Yes, the Propelling lem will always arrive at the top of a successfully created tunnel.

The question now seems to be: what about when the tunnel isn't finished due to the lem hitting a steel block, or the top of the level (i.e. the situations in this image that result in transition to "faller")? Should the lem fall (potentially fatal), or propel safely back down?

I'm currently about 50/50 on it, either would be fine code-side.

My gut expecation would be to fall

lem hitting a steel block
fall (potentially fatal), or propel safely back down?

You can copy what the jumper does: Allow a queued shimmier assignment to execute here, otherwise fall. Then your two upwards-flying skills (propeller and jumper) are consistent with each other.

Feeling: NL/SLX allow climbers to jump off walls, swimmers to jump out of water, ..., then why not propellers to shimmy, too.

Yes, the skills should be about one thing and have few rules (as WillLem says), yet be able to play roles of swiss army knifes (as mobius says). The potential falling (with or without allowed shimmier assignment) has its uses, too: Combine with floater, combine with a higher lemming splatforming underneath in the meantime, ...

-- Simon

I agree with mobius: Most skills get cancelled when they hit terrain they can't affect; This is the logic the first level of Lemmings World Tour tries to teach:
- a destructive skill stops and turns around when it hits steel
- a constructive skill stops and turns aroudn when it builds into terrain

If a Propeller / Twister hits his head on steel, the skill should get cancelled. Thus, even if "propelling down safely" were a feature of the skill, it would no longer work here, because the skill would already have been cancelled by hitting steel, and thus, the lemming would turn into a regular Faller anyway.