Just two comments:
"Observations: ... payload grows only when required" is correct but a more correct explanation would be "the whole list structure is created with a minimum memory usage in mind".

The above note (minimum storage size) leads to special cases:

1) an integer 0 is stored in two bytes only
   02 04     (length, type) and not
   03 04 00  (length, type, data) which is also accepted(*)
   
   The same goes for nullstring (which is obvious)
   02 01     (length, type and, of course, no data), ASCII nullstring
   02 02     (length, type) a "Unicode" nullstring, 
   
2) If only the length component is present and equals 1,
   then this indicates a NULL element (i.e. a missing element):

set x=$lb(85,,,0,"","abc")
zzdump x --> 03 04 55 01 01 02 04 02 01 05 01 61 62 63

which breaks down into
03 04 55         $li(x,1) = 85
01               $li(x,2) = <NULL VALUE>
01               $li(x,3) = <NULL VALUE>
02 04            $li(x,4) = 0
02 01            $li(x,5) = "" / nullstring
05 01 61 62 63   $li(x,6) = "abc"

(*) I use this side effect (list use minimal bytes for integers) and ASCII strings are accepted even if their type is unicode) in some of my CallOuts to return results (as an IRIS-List) without explicitly converting C's two byte string into one byte ASCII where it aplies and I return integer values either as four or eith bytes even if the value would fit in two, three or five bytes.

For example, the ReadColumn() method of the excel library class could return something like

set colData = %exl.ReadColumn(3)

zzdump colData
08 02 61 00 62 00 63 00 06 04 55 00 00 00   (ASCII "as" Unicode, 1 byte integer in 4 bytes)

zwrite colData
colData=$lb("abc",85)

Thank you guys at ISC for this wise implementation!
 

In the history of mankind there were three great inventions
- to light a fire
- the wheel
- and of course, the ^globals

There is nothing more to say.
 

First, there is no NaN in JSON.

set dynObj = {"val":($double("NAN"))}

write dynObj.val + 123 ---> "NAN

zwrite dynObj

dynObj={"val":($double("NAN"))}  ; <DYNAMIC OBJECT>

/* Question @devcommunity manager: how to exit the "insert code block" function? */

Second, red this for more understanding

I'm not sure, do I understand you correctly... 
If you had a class and copied that class to a new class with some more properties, something like this

Class DC.OldData Extends %Persistent
{
Property FirstName As %String;
Property LastName As %String;

/// Redirect the %Open(), %OpenId() to the new class
///
ClassMethod %OnDetermineClass(oid As %ObjectIdentity, ByRef class As %String) As %Status [ ServerOnly = 1 ]
{
	Set class="DC.NewData"
	Quit $$$OK
}

/// Prevent creating a new instance of the old class
Method %OnNew() As %Status [ Private, ServerOnly = 1 ]
{
	Quit '$$$OK
}
}

Class DC.NewData Extends %Persistent
{
Property FirstName As %String;
Property LastName As %String;
Property MartialStatus As %String;
}

Maybe you send your new companion back to the school or maybe better, you look for a real companion with brain.
A docu for return is here By the way, did you know, AI is the shortcut for... no, no, it means Absent Intelligence!

Assuming, your XML stream never has more  then MAXSTRING characters AND if you can live with a "quick-and-dirty" solution then try this one

Class DC.XML.abc Extends (%RegisteredObject, %XML.Adaptor)
{
Property def As %String;
Property ghi As %String;
Property jkl As jkl;
ClassMethod Test(str)
{
    if $isobject(str) { set:str.Rewind() xml=str.Read(str.Size) } else { set xml=str }
    for{set i=$f(xml,"<![CDATA[",$g(i)) q:'i
        set j=$f(xml,"]]>",i) zt:'j "XMLE"
        set $e(xml,j-3,j-1)="", $e(xml,i-9,i-1)="", i=i-9
    }
    
    set rdr=##class(%XML.Reader).%New()
    if 'rdr.OpenString(xml) write "OpenErr",! quit
    do rdr.Correlate("abc","DC.XML.abc")
    while rdr.Next(.abc,.st) { zzdo abc }
}
}

Class DC.XML.jkl Extends (%RegisteredObject, %XML.Adaptor)
{
Property mno As mno;
}

Class DC.XML.mno Extends (%RegisteredObject, %XML.Adaptor)
{
Property pqr As %String;
Property stu As %String;
}

And some tests...

set s1="<abc><def>010203</def><ghi>040506</ghi><jkl><mno><pqr>070809</pqr><stu>101112</stu></mno></jkl></abc>"
set s2="<abc><def>010203</def><ghi>040506</ghi><jkl><![CDATA[<mno><pqr>070809</pqr><stu>101112</stu></mno>]]></jkl></abc>"
do ##class(DC.XML.abc).Test(s1)
def................................: 010203
ghi................................: 040506
jkl.mno.pqr........................: 070809
jkl.mno.stu........................: 101112
do ##class(DC.XML.abc).Test(s2)
def................................: 010203
ghi................................: 040506
jkl.mno.pqr........................: 070809
jkl.mno.stu........................: 101112

Note: the above ZZDO command takes an oref as argument and prints it, you can replace it with a simple zw oref.

You do not need to reference 'localhost', your C++ program and IRIS are located (and running) on the same 'localhost', the source of your problem is something else

Excel store date values as decimal values (or better, as flotaing poin) as dddd.tttt where ddd is the number of days since a base date (usually 1899-12-30) and tttt is: numberOfSecondsSinceMidnight / 86400, hence the value, you get is an date-object, and you have to format it according to your needs

Class DC.PyExcel Extends %RegisteredObject
{

ClassMethod Test(fn = "/home/kav/test/readtest.xlsx")
{
	set exl=##class(%SYS.Python).Import("openpyxl")
	set wbk=exl."load_workbook"(fn)
	set sht=wbk.active
	
	for row=1,2,3 {
		for col=1:1:3 {
			set cel=sht.cell(row,col)
			set typ=cel."data_type"
			set val=cel.value
			
			write ?col-1*15,$case(typ, "s":val, "n":$fn(val,",",2), "d":val.strftime("%a, %d.%m.%Y"), :"")
		}
		write !
	}
}

}

At least, there are four different ways to get that info

ClassMethod StudioTest()
{
	write "Call from Studio1: ",..InStudio1(),!
	write "Call from Studio2: ",..InStudio2(),!
}

/// Is the invocation from the Studio?
ClassMethod InStudio1()
{
	for i=$st:-1:1 if $st(i,"PLACE")["%Studio.General." ret 1
	ret 0
}

/// Is there a connection to Superserver?
ClassMethod InStudio2()
{
	set port=1972 // see iris.cpf, section [Startup], DefaultPort=...
	quit $p($v(-1,$j),"^",3)[("|TCP|"_port_"|")
}

Compiling routine DC.Util.1
Compilation finished successfully in 0.024s.

do ##class(DC.Util).StudioTest()
Call from Studio1: 1
Call from Studio2: 1


ICINDY:USER>; from PuTTY   -------------------

ICINDY:USER>do ##class(DC.Util).StudioTest()
Call from Studio1: 0
Call from Studio2: 0

The other two methods are: checking for the presence of
a) a dedicated variable or
b) a dedicated object
but both require a usage of undocumented functions

I can't reproduce the problem  

Try it this way...

Class DC.OldStuff Extends %Persistent [ StorageStrategy = NewStorage1 ]
{

Property Rec As %Integer [ Identity ];
Property Name As %String;
Property City As %String;
Property Phone As %String;
Storage NewStorage1
{
<SQLMap name="Map1">
<Data name="City">
<RetrievalCode>s {*}=$zel(^myGlo("R",{L2}),2)</RetrievalCode>
</Data>
<Data name="Name">
<RetrievalCode>s {*}=$zel(^myGlo("R",{L2}),1)</RetrievalCode>
</Data>
<Data name="Phone">
<RetrievalCode>s {*}=$zel(^myGlo("R",{L2}),3)</RetrievalCode>
</Data>
<Global>^myGlo</Global>
<Subscript name="1">
<Expression>"R"</Expression>
</Subscript>
<Subscript name="2">
<Expression>{Rec}</Expression>
</Subscript>
<Type>data</Type>
</SQLMap>
<StreamLocation>^DC.OldS</StreamLocation>
<Type>%CacheSQLStorage</Type>
}

}

A short test shows, it works

USER>k ^myGlo
USER>s ^myGlo("R",1)=$zlp("John,Boston,11-22-33")
USER>s ^myGlo("R",5)=$zlp("Laura,New York,333-444-555")
USER>s ^myGlo("R",7)=$zlp("Paul,Chicago,556-666-777")
USER>d $system.SQL.Shell()
SQL Command Line Shell
----------------------------------------------------

The command prefix is currently set to: <<nothing>>.
Enter q to quit, ? for help.
USER>>select * from DC.OldStuff
3.      select * from DC.OldStuff

ID      City    Name    Phone   Rec
1       Boston  John    11-22-33        1
5       New York        Laura   333-444-555     5
7       Chicago Paul    556-666-777     7
3 Rows(s) Affected
statement prepare time(s)/globals/lines/disk: 0.0003s/5/159/0ms
          execute time(s)/globals/lines/disk: 0.0003s/13/1136/0ms
                          cached query class: %sqlcq.USER.cls43
---------------------------------------------------------------------------
USER>>quit

or as objects

USER>s obj=##class(DC.OldStuff).%OpenId(7)

USER>w obj.Name,!,obj.City,!,obj.Phone
Paul
Chicago
556-666-777
USER>