#1)
- create a simple Business Process using the wizard
- compile it and see the resulting class and at least 4 related .int routines.
- then decide if you really want to do this all manually
the challenge is to properly use and service the methods you see in Ens.BusinessProcess

To make it easier you may install (or activate) the ENSDEMO namespace as an example (before IRIS. have no idea where it is gone)

#2)

you write.    ---transformation (written in DTL), but the code is manually written. ---

It might sufficient to put your code just into a CODE Block of a BP generated with the wizard.

There's still alert.log - (not a big hope)
eventually, a Windows event logging may give a hint. - (less hope)

next emergency startup with -nostu

WRC = last chance

Does  %installdir%/mgr/cconsole.log have anything useful to tell you ?

Terminal Multi-Line Command Editor will do it

See also this Editor Archaeology  as long as you don't use WebTerminal.

or my new Terminal Multi-Line Command Editor

Pls. use GitHub.
That makes it easier to follow and complete

Is it really %String or a subtype without MAXLEN or MALXEN disabled ?
I have done this in past and I know of some types just defined to remove MAXLEN =50

e.g. %Library.RawString, %Library.Text, ...

in class %Dictionary.CompiledProperty you have

if  a Property Parameter exists it is in this Array:  
But not all properties have MAXLEN  this is a %String

exact!  
ZS temp creates temp.int

If you call routines or classes you should temporarily save your lines before execution
e.g.  ZS temp

use local routine. like:

to edit/show  your line use ZI, ZR, PRINT  

ATTENTION: you need a REAL terminal. 
WebTerm, Studio, .... just can process  single lines as they are only Terminal SIMULATIONS using eXECUTE command

Glad I could help! It was tricky somehow. 

Great idea !

ONE -> MANY but MANY is UNIQUE