Maybe, I think that could all happen. I think there would be a lot more of the Riker-Deanna stories on TNG with a Riker captaincy and perhaps they get married earlier, maybe in the TV show's run instead of in the last TNG movieIn universe I do not think they would have killed Picard, he would have had his Heroic moment to overcome the Borg programming and tell Data how to defeat the Borg. Afterwards he would have been retired from the Enterprise to head up an 'anti-Borg taskforce' so they can use Picard again at some point. Probably allows Shelby to take the XO position when Riker steps up as Captain.
TNG would retool for a Riker Captaincy, but most the stories could do ahead. Maybe more Picard orientated episodes like The Chase would not happen or be changed. There might be a quality drop off without Stewart, but the show will probably work that out, maybe becoming more 'actiony' than OTL?
Steward himself would have come back to Britain and done RSC stage work, and probably TV/movie as they interested him. Portraying Xavier is still highly likely imho. I certainly do not think his career would have suffered. It seems likely he would be back on Trek in one way of another before the end.