holch wrote: But, in this case I don't think it will work without tokens and token based answer persistence. Because how long do you want to keep the session open for this survey?

As long as you want to. When doing B2B surveys with sales people a session of 12h is quite helpful, when people are often on the phone or elsewhere.

But the example with the train and the certain course ID is not the ideal example.
You won't want to let people type in an ID in the first place.
When asking for things the respondent might not know, I would put the requirements in the invitation and on the first page.
When using token/answer persistence, I would put the difficult questions at the beginning in a group and let people come back and enter them at the beginning and then skip to the end again to confirm data via submission again.

All this wouldn't change the fact, that a question setting is a question setting. So mandatory is mandatory. The academic discussion about "moving on" vs. "submitting" can be fruitful for survey design, but wouldn't change the feature of what a "mandatory" question is.