1) Never let the battery get below 40%. That's not Google, that's lithium batteries. You're cutting the battery life in half by discharging to 20%. With a tablet, a dead battery means a trip to the shop, labor costs and a more expensive battery than a phone you can just pop the battery out of. (They charge more because it has a few cents of parts added - and because they can.)
2) The recommended charging time for a lithium battery is 0.75C, or 3/4 the capacity of the battery. Adding in inefficiencies (no energy conversion process is 100% efficient), the fastest you can put a full charge into a lithium battery without endangering it is 1 hour. Taking 2 hours extends the life a bit. (With proper care, a lithium battery can last 10 years. With improper care, you can completely kill it in 6 months.)
3) One of 2 things will happen if you use the phone charger on the tablet:
a) If the phone charger can't supply as much current as the tablet wants to draw, it'll take even longer to charge.
b) If the charger can supply at least as much current as the tablet wants to draw (it can supply 10,000 Amps, it won't make any difference - I charge my phones from a 30 Amp supply), the tablet will charge in the same amount of time it does now - assuming that the charger you're using supplies at least as much current as the tablet wants to draw.
(Where did your husband get his electrical engineering degree? Dental school? They didn't teach what he's claiming when I got mine. Maybe they invented a new kind of electricity since then.)
4) If the tablet is running when you're charging it, some of the "charging" current is actually going to run the tablet, so there's less for the battery, so it takes longer to charge. The more the tablet uses, the longer it takes to charge.
The tablet is designed to draw a fixed amount of current to charge the battery, you can't change that. And if you're charging the tablet when it's turned off, it doesn't matter what version of Android is in it - it's not running, so it won't affect the charging time. (If you haven't cleared the system cache - that's in Settings, in Storage - press Cache and it'll ask if you want to clear it - do it now. An update clears cache bit by bit and that chews up a lot of power. Doing it all at once is more efficient,)