In general you're asking for the Pace Pro Course creation tool in Garmin Connect (or some other Pace Pro Course creation tool) to dynamically adjust pacing strategies in one or multiple ways. I think this is very possible.

One of my complaints when I first read about PP was that it appeared to be a product and not a platform. DCR himself chimed in to reply that is incorrect. PP as it is implemented today is a static ruleset applied to a course through the Garmin Connect app. However the framework itself is not so rigid. Garmin Connect could, in theory, use any sort of data you can dream of to set targets on a split by split level in a course. It is also possible that other sidecar data could get attached to the course (like perhaps a reminder to consume a gel @ splits 10 and 20).

It would also be possible for other parties to write the Pace Pro Course file...

So I think your 3 goals are within reach, but maybe not implemented on the watch itself. At lest not this generation of hardware.

As to some of your other comments:

Runners tend to split their races not only in distances or changes in elevation but also between aid stations. Maybe allow for waypoints in a gpx file to serve another split indicator

I think this comes down to granularity of the PacePro'd Course. Based on the fact Garmin plans on this being open there's no reason some other app (if not GC itself) won't allow arbitrary split creation (instead of just mi / km / hill).

The slider that controls the effect of uphill effort doesn't go far enough, so minimum uphill paces are too fast (or vice versa, downhills are too slow). Walking uphill to conserve energy is not uncommon even among strong athletes, plus highly technical trails will slow that down even further.

I believe this is just an artifact of a 1st generation "Apple-like" PP Course creation tool. What you're describing here is a want for things other than elevation to automagicly adjust pacing strategies. (I know that hill is slick shale and I'll be slow, I know that slope is smooth dirt and I'll be fast, I know that descent is rooty and after dark and I'll be really slow lest I trip). Adding the ability to manually place split points and manually tweak the AI's guesstimates wold solve this.

Strategies seem to be limited in the total duration or distance, which seems counterintuitive when especially mountain races may take longer than similar distances on flat roads.

I don't know enough to comment on this.