I've got an Altimax. I use the altimeter and logbook fonctions often all day in mountain for approx 100 days/year.
The battery life is about 2 years.
I try once to skydive in a protek. I set the altitude alarm to 4000 ft and it sounds when i was under my canopy at 2500 ft...
The instructions manual says that it refreshs (takes measure) altitude up to 10s.
In logbook mode it'll be mini 20s, that why i doubt you coul have an accurate fall rate if you stop it under canopy.
About the "ski runs" I think you 'll have to reach exactly the same altitude to count a lap, like with a chair lift which "did not move" too much.
The only advantage i can see is, maybe, under canopy to have an accurate altimeter (15ft).
To see how many dives you have done, for the same price buy a (2nd?) pro-track.