Alot of questions and I am not familiar with the clematises that you mention but there was a possibility that your Henryi didn't die. Sometimes young plants disappear the following season and may not emerge again for a year or even more - at least according to a number of people on the clematis forum. For my own (a "Prince Charles" that is going into its 4th year for me), it is a group 3 so for winter, I leave the vines on its support and in late February, I'll go out and cut it back to the first couple pair of buds. Often the rest of the vine has some buds trying to swell during late winter warm spells, but I still cut the whole thing back because it will inevitably engulf the support by bloom time. When/how often it is recommended to water during winter - that is mainly if it is dry (no rain or snow) and/or in my case where mine is on a covered balcony and happens to be against my bedroom wall and not along the rail to really get much natural water unless we have a Nor'easter (where I face NE and get the full brunt of those windy wet Atlantic coastal storms). One would water when the soil was dry and the temps were well above freezing - ie., in the 40° Fs or higher. In general, plants need less water during the winter when dormant, but they do appreciate some for their roots as the roots continue to grow in winter when the soil is not frozen. Yes, the containers can often freeze, but the bigger it is, the longer it would take to do so. For hardy perennials, this isn't so much of a problem as a detrimental cycle of freeze/thaw, which can burst the roots. So once it's frozen, it should hopefully stay that way and graduallly thaw. If you get snow, then that is a good insulator and you can put some mulch on top (I use pine needles on my clematis). Only thing you can do is try. You might have a mild winter that will allow a young plant to establish a more mature root system and gradually gain its fully hardiness in a container. I am finding that this is often the case with plants - they need time to gain that hardiness (even if planted in the ground and moreso in a container). The one other thing to look out for is the phenomena where containers often heat up faster than the ground, so sometimes this triggers your plants to wake up a little earlier than they should and then along comes a frost or freeze. Clematis buds are somewhat sensitive to sudden freezes if they are swollen or leafing. This was my clematis at the end of last June: The ridiculous below-normal temps makes me think it ain't gonna be ready to bloom earlier than normal like it did last year! Especially since it looked like this on 5/23/04: - and the stems are barely 2ft or so tall right now:

I (try) to control the amount of water the pot receives in fall when it's rainy, but I forget or I'm lazy. If the soil gets too wet, I shove one end of a strip of cotton fabric, (like from an old towel) into a drain hole about 2 inches, and let the rest of the strip of fabric hang out of the drain hole. I keep the pot raised up so the strip can hang down. The fabric wicks excess water out of the pot. Once the temps are reliably below freezing during the day - I put the pots into the garage. I keep a little snow on top of the pots so the surface doesn't dry out during winter. I keep the pot in the garage until the soil thaws and we're getting rain and not hard freezes. This year I'm going to keep a strip of fabric in all my pots so I don't have to worry about over-watering.

Lucky you!! B or S is right about needing to bury pots to protect them from temperature extremes and freeze thaw cycles. I have found that if I bury the whole pot in soil, I often have drainage difficulties when the soil in the pot freezes and then just the surface melts, floods and refreezes during what is referred to as "mud season" in this part of the country. I guess if you have a slope you could bury them in soil there so that any melt water will drain away. I usually bury pots to the rim in mulch that won't pack down, most often wood chips or shavings since it seems to allow better drainage. I usually give the surface of the pots a lighter layer of mulch as well. I've heard of other folks putting all their pots in one area and surrounding them with bagged leaves. Since I live in a really cold area where it rarely thaws until spring, my goal is to keep things from heating up and going through more freeze-thaw cycles, so I usually put them where they have a northern exposure and so get no direct sun. I've heard others in slightly warmer areas who place them near a warm foundation to prevent freezing. I guess you have to figure out what will work best for you to prevent freeze-thaw cycles which are likely to damage your plants.

The temperature is more important than light. You want to keep the plant as cool/cold as possible in your basement, 32-50F would be ideal. You don't want to overwater, but give it a bit of water once every 3 or so weeks. Light isn't necessary if it's truly dormant. Then in the early spring, cut it back and get it outside, gradually exposing it to light.