Yeah, I know, not exactly the least discussed episodes around here, but still 
1) What did Guinan actually know about the Borg?
First, she tells us that "They're made up of organic and artificial life which has been developing for thousands of centuries. " This seems (but isn't necessarily) in contradiction with what we learn later about them. Even so, if this is true, how did she learn about this? There aren't many races that are offered a glimpse in the history of the Borg, and they're not exactly from Guinan's neighbourhood, either. Or can we just ascribe this to her "trans-reality" perceptive powers?
Secondly, she strongly advises against Riker visiting the Borg cube. Which is kind of weird, as even though the Borg are aggressive, individual visitors are usually ignored (and she probably would have known that). In the end, it seems to have been a good decision, too (as they learned a lot they didn't know yet). Would Guinan have known about the danger of assimilation? Or did she just know enough to be very, very afraid but not more than that?
(I know, assimilation wasn't even a concept when 'Q who' was made, but I'm just trying to think in-universe).
2) Could Picard have destroyed the cube at the beginning of Q who?
Yeah. I know, at the end of the ep Q says "You can't outrun them. You can't destroy them. If you damage them, the essence of what they are remains. They regenerate and keep coming. Eventually you will weaken, your reserves will be gone. They are relentless.", and Federation weapons did prov to be near to useless, and in BOBW as well.
Still, in just a few seconds he blew a few mightily impressive holes in that cube earlier in Q who. That's after the Borg had sent a scout, mulled over the gathered data for 20 minutes or so, and said 'we have analysed your defensive capabilities as being unable to withstand us'. They hadn't adapted to the Ent-D's weaponry yet, seems they just made a general assessment and didn't bother with the details. So, suppose Picard would have gone all-out at that very instant, could he have blown up the cube, or would the Borg still have adapted quicker than he could fire ?
For that matter, suppose Picard had been briefed by Q or Guinan beforehand, and started firing on the cube even before that drone came aboard, would that have made any difference?
3) Would BOBW have worked as well, had Shelby been replaced by a young and very ambitious Lt. Cmdr. Janeway? At the danger of being a serious case of 'small universe syndrome', I could see it setting up a nice rivalry/bond between her and Riker, and give her a background in Borg knowledge at the same time. Would these two characters have played nicely off each other?

1) What did Guinan actually know about the Borg?
First, she tells us that "They're made up of organic and artificial life which has been developing for thousands of centuries. " This seems (but isn't necessarily) in contradiction with what we learn later about them. Even so, if this is true, how did she learn about this? There aren't many races that are offered a glimpse in the history of the Borg, and they're not exactly from Guinan's neighbourhood, either. Or can we just ascribe this to her "trans-reality" perceptive powers?
Secondly, she strongly advises against Riker visiting the Borg cube. Which is kind of weird, as even though the Borg are aggressive, individual visitors are usually ignored (and she probably would have known that). In the end, it seems to have been a good decision, too (as they learned a lot they didn't know yet). Would Guinan have known about the danger of assimilation? Or did she just know enough to be very, very afraid but not more than that?
(I know, assimilation wasn't even a concept when 'Q who' was made, but I'm just trying to think in-universe).
2) Could Picard have destroyed the cube at the beginning of Q who?
Yeah. I know, at the end of the ep Q says "You can't outrun them. You can't destroy them. If you damage them, the essence of what they are remains. They regenerate and keep coming. Eventually you will weaken, your reserves will be gone. They are relentless.", and Federation weapons did prov to be near to useless, and in BOBW as well.
Still, in just a few seconds he blew a few mightily impressive holes in that cube earlier in Q who. That's after the Borg had sent a scout, mulled over the gathered data for 20 minutes or so, and said 'we have analysed your defensive capabilities as being unable to withstand us'. They hadn't adapted to the Ent-D's weaponry yet, seems they just made a general assessment and didn't bother with the details. So, suppose Picard would have gone all-out at that very instant, could he have blown up the cube, or would the Borg still have adapted quicker than he could fire ?
For that matter, suppose Picard had been briefed by Q or Guinan beforehand, and started firing on the cube even before that drone came aboard, would that have made any difference?
3) Would BOBW have worked as well, had Shelby been replaced by a young and very ambitious Lt. Cmdr. Janeway? At the danger of being a serious case of 'small universe syndrome', I could see it setting up a nice rivalry/bond between her and Riker, and give her a background in Borg knowledge at the same time. Would these two characters have played nicely off each other?