I'll try to answer your questions from my experience of weekly sailing a 16ft dinghy
Firstly, how much the boat heels & how quickly before capsize is very dependent on the dinghy in question. Some of your dinghy skiffs are very sensitive and take men/women far fitter than me to sail. Other dinghys, like the Corsair 16ft our family takes out on the weekend, are pretty solid and will only capsize if you let them or push them too hard or in too strong a wind.
I am not the world's best dinghy sailor, in fact I'd probably rank around the bottom 20%. That said, I am more than comfortable taking our 16ft dinghy out in 20 knot winds without fear of capsizing her. I'd be avoiding more risky maneuvres and sailing a little less close to the wind on the work, but it's not only possible - guys like me can do it
I've seen more competitive sailors take the same class dinghy out in over thirty knots, but while they finished the race - they did so after being dunked several times and at least one of them came back with a broken boom.
I take my wife out with me whenever I can and she's eager. Neither of us like having to swim when we're supposed to be sailing so we don't capsize. That said, it is not a bad thing to capsize with your partner at least once so she understands it's not a big deal. I know that once I went through that with my wife, she became far more comfortable with heeling as she realised the worst that would happen wasn't really that bad. Getting a dinghy that size back up is a piece of cake with one person, even easier with two.