[Having just re-watched the episode]
a) Teal'c and Sam - the whole of Teal'c's technical knowledge prior to breaking from Apophis was theoretically that it was Goa'uld magic - he tells them (in COTG?) that they were forbidden to know how such things worked. BUT he must have picked some stuff up, and as mentioned above, he is clearly far from stupid, and working around Sam and Daniel seems to be learning fast. Sorry Jack-fans, he is not that sort of thinker!
b) Fate of P2A-870 - I am missing so much by being in Britain without satellite feed, and hence only being at the end of Series 3!!
c) Other things that came to me:
+ is there really all the strength in the iris to support the time it stayed in place - those irises are not designed for strenth but to provide a 100% closure, e.g. cameras, and on the gate, it is merely to stop reintegration of matter, so do not inherently need lateral strength against actual mass/force.
+ I think Jack's question was the writers making sure the audience still had the plot, though it gives good character filler for the two protagonists.
+ gravity / lens effect: thinking about it a lens doesn't necessarily get hot by focusing light, so the gate wouldn't necessarily get acted upon by the gravity coming through the wormhole
+ the gate should still close at the Maj. Boyd end sometime. Even if they only got six frames in eleven minutes or whatever, that is still 1/4 second, and therefore they only need 152 * 60 more of eleven minutes... though it would presumably slow down more