There arises two questions.

First, why do you use a class where the documentation starts with "This class is used internally to hold property/field values needed for computed fields"?
Another word for classes which are marked as "used internally" is, that this classes can be changed (altered in its behavior) without any notice or documentation in one of the next versions. That's definitelly not what you want.
 
Second, what is your real problem (where you think, it's only solvable by using that %Library.PropertyHelper class)? Can you post (and describe) your real problem?

Two notes to your (above) code

- first, if you use the iterator method on an object which can contain long strings (longer what IRIS can handle) then you must specify the thrid argument to the %GetNext() method too, i.e.

while iterator.%GetNext(.key, .value, "stream") { ... }

- second, if you know the name and location of a stream property in an JSON object, like in your case, then just grab the data without using an iterator:

try { set jsonobj = {}.%FromJSON(httprequest.HttpResponse.Data) } catch { jsonobj=0 }
if jsonobj, jsonobj.statusCode = 200 {
   set pdf=jsonobj.value.labReport.%Get("pdf",,"stream<base64")  // according to the picture you provided
   ... // do something with the pdf-stream
}

For another example, how to work with long strings see this thread.

You have right, do {code} and do {code} while expr are mutually exclusive.

I thought more something like this do {{code}} (no spaces between curly braces) or, as you suggested, a new keyword like WRAP {...}, of course, BLOCK {...} and SCOPE {...} are also acceptable.

By the way, we already have an alternative for argumentless DO
do {code} while 0
It looks ugly and confusing and
- does not preseve $T
- does not establich a new stack level
but one can get over both easily respective do a work-around.

I hope someone is already working on that extension and it's scheduled for the upcoming release

Oh, I see, at the very first use of a (new) private global,  the storage is decremented (on my  machine) by 768 bytes. I think, that bytes will hold some management data for each provate global (name).

Instance: cindy:ICINDY
Version : IRIS for UNIX (Ubuntu Server LTS for x86-64) 2021.2 (Build 649U) Thu Jan 20 2022 08:49:51 EST

Username: kav
Password: ******
USER>s x=$storage

USER>set x=$storage, ^myKav=123 write x-$storage
768
USER>set x=$storage, ^myKav2=123 write x-$storage
768
USER>set x=$storage, ^myKav3=123 write x-$storage
768
USER>set x=$storage, ^myKav3=123 write x-$storage
0
USER>set x=$storage, ^myKav2=123 write x-$storage
0
USER>set x=$storage, ^myKav=123 write x-$storage
0
USER>

I don't see any difference

USER>write $storage,! set ^myTest=123 write $storage
2199022714000
2199022714000
USER>write $zv
IRIS for UNIX (Ubuntu Server LTS for x86-64) 2021.2 (Build 649U) Thu Jan 20 2022 08:49:51 EST
USER>

Try the above line. Maybe you have issued one or more "set variable=..." commands between the two "write $storage" statements in your tests...

Just a curious question, do you talk about a form with encoding? Something like this

<form method="post" enctype="multipart/form-data" ...>
...
</form>

If yes, I think WRC will be your friend. In any case, I have never worked with such type of encoding. Maybe someone else?

And you are certain,  the correct name is "profile"?

Maybe you didn't read it carefully enough...

/// the documentation say clearly:
///
/// %request.Data(itemName, itemIndex)
///
set vProfile1 = $Get(%request.Data("profile",1)) // the first value
set vProfile2 = $Get(%request.Data("profile",2)) // the second value
/// etc.

/// or you use a loop
///
kill value
ser value=0
set idx=$order(%request.Data("profile",idx))
while idx]"" {
    set value($increment(value))=%request.Data("profile",idx)
    set idx=$order(%request.Data("profile",idx)
}

/// value = item count
/// value(i) = i-th value

See the class documentation

A few lines of code and you have your own object cloner

Class DC.ObjCloner [ Abstract ]
{
/// obj: Cache/IRIS or a Dynamic(JSON) Object
/// 
/// For simplicity, JSON-Types null, true, false, etc. are ignored
ClassMethod Clone(obj)
{
    if $isobject(obj) {
        if obj.%IsA("%DynamicAbstractObject") {
            if obj.%IsA("%DynamicObject") { set new={},arr=0 } else { set new=[], arr=1 }
            set iter=obj.%GetIterator()
            while iter.%GetNext(.key,.val) {
                set:$isobject(val) val=..Clone(val)
                do $case(arr, 1:new.%Push(val), :new.%Set(key,val))
            }
            quit new
            
        } else { quit obj.%ConstructClone(1) }
    } else { quit "" }
}
}

Class DC.Import Extends (%Persistent, %JSON.Adaptor)
{

Property thingone As %String;
Property thingtwo As %String;
ClassMethod Test()
{
	// create a test stream
	set myStream=##class(%Stream.TmpCharacter).%New()
	do myStream.Write("[{""thingone"":""Red"",""thingtwo"":""Green""},{""thingone"":""Blue"",""thingtwo"":""Yellow""}]")
	
	// convert input (JSON) stream into a JSON object
	set data={}.%FromJSON(myStream)
	
	// loop over the JSON-Array and import each array element into your database
	for i=0:1:data.%Size()-1 {
		set obj=..%New()
		set sts=obj.%JSONImport(data.%Get(i))
		if sts set sts=obj.%Save()
		if sts continue
		write "Error: i=",i,", reason=",$system.Status.GetOneErrorText(sts),!
	}
}

Of course, instead of "myStream" you should use your REST-input stream.

do ##class(DC.Import).Test()

zwrite ^DC.ImportD
^DC.ImportD=2
^DC.ImportD(1)=$lb("","Red","Green")
^DC.ImportD(2)=$lb("","Blue","Yellow")

Shouldn't be a problem, $ZTZ isn't the only thing you can change.

Whatever value or setting you change, $namespace, $zeof, $horolog, etc. you have to consider a simple rule, if you change things (which can be used by subsequent routines, methods, etc.) for your own use, then after using them, you have to restore them to their original value. That's all.

YouRESTorWHATEVERmethod()
{
  set oldZTZ=$ztz
  set $ztz=<your preferred value>
  ...
  set $ztz=oldZTZ
  return
}

You can change the Timezone system variable

//  for example EST (Eastern Standard Time, 5 hours behind UTC)
    set $ztz = 300
//  you can even consider daylight saving
//  for example CET (Central European Time, 1 hour ahead of UTC)
#define DSTOFFSET	-60
#; -60=CET, 300=EST
#define LOCALZONE   -60
    set $ztz = $$$LOCALZONE+$s($system.Util.IsDST():$$$DSTOFFSET,1:0)
    

Setting $ZTZ affects the current job only (the job, which sets $ztz), so there is no danger for other processes.

If you want, for whatever reason, to compute the CRC of a Stream then write an short method like this (or use this):

/// Input : str - a sting or a stream
///         typ - CRC-Type (0..7, See ISC documentation)
///         
/// Output: crc-value
/// 
ClassMethod CRC(str, typ = 7)
{
    i $isobject(str),str.%IsA("%Stream.Object") {
        d str.Rewind() s crc=0
        while 'str.AtEnd { s crc=$zcrc(str.Read(32000),typ,crc) }
        ret crc
        
    } else { ret $zcrc(str,typ) }
}

Where is the problem?