I'll give up on the numbers then!
I'll watch the video later when I can concentrate on it.
The longest climb that I have done is the Tudons climb near Benidorm. That only goes up to ~1,000 metres but it does it from sea level. I think it was about 20 kms (12-13 miles) in length. I did get into a rhythm for it but my back muscles started to complain about halfway up. Unsurprising really because I went out there after not riding my bike all winter and then just threw myself into hilly/mountainous riding - 1,200 km (750 miles) in 2 weeks.
The highest temperatures were 'only' 25-30 C (it was in March) but that was plenty hot enough for me. I would really struggle doing long climbs in temperatures higher than that.
Getting back to the 100 miles/10,000 ft question ...
Err on the side of lower gears rather than higher. If you don't need your bottom gear(s) then you don't have to use them. If you need them but don't have them, then you'd have a big problem!
You need to be comfortable on your bike. If your position is wrong, your saddle is uncomfortable, or your shoes a poor fit then that ride will certainly let you know about it!
Pace yourself well. Better to start off a bit slower and speed up later than start off quick but eventually crack.
Eat and drink plenty.
Make sure that you are not overdressed for hot conditions but carry some extra clothing for long descents and/or in case the weather turns nasty.