set sc=rs.Execute("a")

You pass a paramter,
but there is no need for a parameter visible.  NO ? in query

try

set sc=rs.Execute()

Just an idea:
wouldn't CopyFromAndSave(source As %Stream.Object) be a better approach ??

Httprequest.EntityBody is a %GlobalBinaryStream object.
In your code I see

Do Httprequest.EntityBody.Write(mystring)

But no %Save()  to take care of completion and persistence of the object

Congrats!
By manual intervention, you managed to get your index out of sync with your date.
Check this Repairing your Index

Sorry the method is not yet available in 2014.1 you need a Rebuildindex (from SMP)

Do you expect to save #1) the input JSON object or
#2) a dump of the converted result?
For both:

set stream=##class( %Stream.GlobalCharacter).%New()

#1)  with myjson as your JSON input

do stream.Write(myjson)
do stream.%Save()  

#2) dump the generated Object  obj  using my package Full-OBJ-Dump described here

do ##class(Z.obj).dumpToStream(.stream,obj,1)
do stream.%Save()  

Take a closer look to Caché I/O Device Guide
especially Chapter 5: TCP Client/Server Communication

As you operate on 2014.*  just write it down as any other string.

after 2017.1 you may (but are not forced to) use %DynamicObjects and %%DynamicArrays.
but the result looks the same 

Status= 1 justteslls you tht the HTTP connection was processed OK.
Next you need to take a look into your response object.

set res=Httprequest.HttpResponse
in terminal then ZW res to see in the Status is 200 OK
otherwise what you see I  res.Data might be just an error page.
Which is also OK for HTTP but not for your content.

• property ReasonPhrase as %String;

This is the human readable reason that goes with the StatusCode.

• property StatusCode as %Integer;

The HTTP status code. This is useful to determine if the request was successful. Look in the rfc for HTTP to see which codes are supported and what they mean. A human readable form of this code is stored as the ReasonPhrase

• property StatusLine as %String;

The HTTP status line. This is the first line of the response and signals if the request was successful or if there was a problem.

I found %DynamicObjects first in Caché 2017.1 Class Ref
https://cedocs.intersystems.com/ens20171/csp/documatic/%25CSP.Documatic.cls?&LIBRARY=%25SYS&CLASSNAME=%25Library.DynamicObject

In 2014 this was somehow buried in %ZAN.Auxilary.???

You might take a look to this article Global archiver - Moving a part of a global.
And the related package in OEX global-archiver

Now with the class definition available, I understand(?) what you are looking for.
I see 2 possible solutions: embedded SQL or an Index on Title
#1

btw: SQLCODE=0 means success. 
and you get the RowId by 

SET RowId=##class(REST.TITLE).TitleToRowId(obj.Title)  ;obj=JSON
SET task.Title = ##class(RESTAPI.TITLE).%OpenId(RowId)
SET book.Title = ##class(RESTAPI.TITLE).%OpenId(RowId) ; recent example

#2
 creating an Index on Title in REST.TITLE.
 but you have all trouble on duplicates, max. string length on that index
 So I'd position it as elegant but rather risky on maintenance