Talking about warp factors and talking about time dilation don't go well together, unfortunately. The Star Trek universe is, as best I can figure out, nonrelativistic.

This is a bit of hand waving (which is OK because we're talking about imaginary sci-fi constructs) but it was my understanding that since warp drive bends space around you, you're not actually going anywhere near the speed of light, and therefore you experience time similarly relative to people at rest.

I'd have to see that on a Minkowski diagram before I believed it made sense.

Two words- Minkowski Compensators.

Just reverse the phase polarity in the coupler coils.

That's what I assumed too. I wonder how they handle the causality problems with FTL communication?

no need for warp in order to travel 20 light years and take 10 months subjective time.

You just need to travel at 99.91% of the speed of light and you'll get a 1/24 time dilation effect.