It's not just regen in HV mode. As Aaron said above, the car will divert any 'spare' ICE energy into the battery whenever it works out that it's not needed to drive the wheels. It's not unusual to see the EV range creep up when accelerating gently in HV. Of course, you'll never 'fill up' in HV mode but I've seen it pick up a couple of miles of EV range over 30 miles of driving.
I've experimented with Charge mode and found it had a negative impact on economy, although I wasn't on a motorway to be fair. I'll keep using HV mode myself, but I'll be interested to hear how you get on with it. Please let us know!
Averaging 100mpg across 10k miles shouldn't be a problem in that scenario, although how far you can go above that will depend on weather throughout the year as that has a dramatic impact on EV range. On a warm, dry day I'll get 32-36 actual miles on EV. On a cold, wet day that will drop all the way down to 16-20 miles. On a journey of 80 miles, that will have an enormous impact on mpg - expect less than 100mpg in the winter months, balanced by considerably more in the summer.