One thing they said and I'm not sure how I feel about it, is that every class can use every weapon in the game, but you can only carry two weapons with you at a time unless you're a Soldier (then you can carry all of them). It might be alright but I can't realistically see anyone bring a Pistol with them unless they buff the Pistol.
That really depends. If your class does not have the skills necessary to use certain weapons effectively (Sentinels, for example, can only use pistols and SMGs pre-upgrade), it makes more sense to bring in a pistol which you can use effectively than an assault rifle which you cannot. I mean, try using an assault rifle as a sentinel in ME1, it's impossible. Besides that, each class outside of Soldier has abilities that are weapons unto themselves (tech and biotics), which can be infinitely more effective than weapons - as an Adept you're pretty much never at a loss, offensive-wise, because even with no ammo left you can throw out shockwaves and singularities and wait out the cooldown timer. It pretty much depends on how they balance weapon usage among classes.
I assume that, like in ME2, there will be a mid-game upgrade that allows you to carry another weapon or upgrade one existing weapon type.
I think I've read somewhere that only successful last mission can be imported to ME3.
How many people can you lose while still being successful?
Technically, 10. Maybe 11.
You need the last two people to hoist you into the Normandy, and I've never had only one person in my party survive.