Please note that we've updated the Mount & Blade II: Bannerlord save file system which requires you to take certain steps in order for your save files to be compatible with e1.7.1 and any later updates. You can find the instructions here.
I don't think so. However that is just my opinion. It's only multiplayer, the most fun I've gotten was from linebattles, However that's because we had a silly group in our Line.
Viking Conquest is pretty good, has it's own interesting story with nice surprises in it, and it has multiplayer (I don't really see people play it that often though). The game is very different from Native since they added alot of things regarding it.
If you join a Regiment in NW (Linebattle) you will see quite a few people playing at once since it's organized but they are usually private events., depending on what connections the leader of the Reg has. As for Warband, I can't give you a definite answer since people are all over the place regarding mods and areas as to where the players are (which can result in low or high ping). If you are wondering as to whether alot of people still play NW, and if you have Warband, I would suggest going to the multiplayer lobby and turning off the module filter and look and see if theres alot of people playing NW. that should give you a good idea. I haven't been keeping an eye out for it as of late. Were you looking for a specific Region you wanted to play with? EU, NA?
My server still gets full at 100 people, especially on the weekends. that's NA. There's still like 4-5 EU Side servers can max out still, some over 100 players easily. Events though are still live and have hundreds of players, almost every night of the week.
NW is definitely worth it! Joining a regiment is tons of fun and I'd highly recommend you try it. The viking conquest DLC is also very good, but mainly focussed on singleplayer while NW is mainly multiplayer.