1) The poles attached to the red army were sent in unsupported, and had little hope of pulling off a relief. In fact, I'm not entirely sure if they were ever ordered to act, or did so on their own (I'll get back to you).
2) The Soviets had enough contacts in the Armia krajowa to know when the rising was occuring; even the germans had a clue that it was coming. And why were the Soviets unable to continue the offensive a few more days and 10 kilometers at most?
3) Not sure about the numbers, but the sources I've seen state that soviet aid arrived to late to make a difference.
But anyway, on to AH...
The polish domestic situation will be interesting for the next few years. The non-communists will find themselves in a strengthened position compared to OTL. The russians will be running things, but this gives the government in exile in London additional credibility, and a foothold on the ground. My guess is that the home army gets sidelined by the russians. they are just to far from western support to avoid that fate. However, if they do hold on and demand a voice in the postwar nation, things get interesting. For one thing, if they can compell the powers to notice them, the entire division of europe springs a few leaks. Not sure where it all leads, though.