1) The GeForce 6100-M7 and TForce 6100-754 are, as you saw, essentially the same motherboard. The TForce is built a bit more solidly, has higher quality transistors and probably a few more features in the BIOS. However, the GeForce 6100-M7 is still a great board, and it has all the necessary overclocking features to get a Sempron chip to 2.3-2.6 GHz.
2) No. Dedicated video is almost always superior to integrated. That is not to say the GeForce has bad graphics, though. In fact, some reviews I've seen rate the GeForce graphics as the best integrated video on the market.
3) Yes, it shares system memory. The lowest possible setting is probably 32 MB. The highest setting could be 64 or 128 MB--you'd have to check on the website to be sure. 64 MB is a decent setting for integrated video. And, yes, it cuts into system memory, but I wouldn't worry about that. It means you'll have 448 MB instead of the full 512, or 896 MB instead of 1GB. No biggie.
4) One bad experience isn't the end-all-be-all of a company's customer service. On the contrary, people seem to be very happy with Biostar boards--especially the GeForce you're considering.