Not 2 sure

1) Yes getting work and accommodation shouldn't be a problem
2)HCMC is more sociable, Hanoi pays more.... take your pic
3) I wouldn't do it given the distances involved but it's certainly doable....

As for accommodation - no problem, plenty, all qualities and prices available.
As for differences Hanoi-Saigon, I personally find Hanoi more social and city like, especially the old town area - but dislike the cold and long rain season and the quite drastic regular flooding with it.

Especially good, in Hanoi I like the much less common double pricing for foreigners as well as the better/more common English, even outside the tourist routes than in Saigon.

As for you  trip from BKK, I think, it's not really a good idea. You still will need a visa and the bus ride is not really that exiting, if you choose Hanoi as destination, even less. You might need a long holiday to recover from your bus trip. Price differences are not really that big, so a plane would be more easy.
Also, there are occasionally some issues with the FIRST ENTRY and the validity of visa via land, why and how, maybe someone else knows or can explain. I only know, there some guys staying longer here, they had quite serious issues and to "convert" their visa to another.