I will check form scripting once more - but for now it looks like I cannot access the document. In the docs there is a sentence about the start form being displayed BEFORE technical start of BPM, perhaps that`s the reason.
Concerning second part:
Every manual step (work item) can be assigned a form. But this form is not presented directly in the ui (yuuvis web client). One has to go to the inbox, klick the process, click "take over work item" and see the form inside the inbox. And after that not forget to klick "continue" (or whatever the English term is here) to continue the bpm.
My hole bpm is just for computing a value and present it to the user. To do all this inbox-thingy just to read a single value is a little much to explain to them. (Most of them not knowing yuuvis yet)