Very strange syntax:

But if we tried at once with a single statment with a select into (1 value) it fails

INSERT INTO Sample.YoungPeople (PASReligionCode) values (SELECT internalPatientNumber FROM
Pennine_TIE_Clinicom_Link.PMISPECIALREGNCA where InternalPatientNumber=100)

See:

• Insert Data from Another Table Using a SELECT Query
• %SYSTEM.SQL.Schema:CreateLinkedTable()
• Defining a Table by Querying an Existing Table
• %SYSTEM.SQL:QueryToTable()^deprecated
• %SYSTEM.SQL.Schema:QueryToTable()^deprecated

There was something similar: Event onPostDispatch %CSP.Rest or similar

Yes.
Or you can immediately increase the logical size of the IRISTEMP to the desired value, for example:

%SYS>set status=##class(Config.Databases).Get("IRISTEMP",.props)
%SYS>set status=##class(SYS.Database).ModifyDatabase(props("Directory"),5*1024) ; 5Gb

In this case, you will immediately increase the size from 240Mb to 5Gb, bypassing many small expansions.