Walkers and overwatch
To start I believe every infantry type model can only fire 1 weapon in overwatch due to most rules like Gunslinger specifying "All models with two pistols can fire both in the shooting phase. This follows the normal rules for shooting."(page 42) Since overwatch takes place in the assault phase then that rule doesn't apply and you only get 1 shot with pistols in overwatch.
In regards to walkers:
The walker section says "A Walker that moved can still fire all of its weapons in the subsequent shooting phase." (pg. 90) and later on page "Furthermore, Walkers can fire their weapons even if the charging unit is not within their current weapon arc, ".
So it does use weaponS (plural) in the second quote, but otherwise I don't see much different between infantry and walkers. Is this plural form of weapon enough to enable all weapons for walkers in overwatch?
So do walkers get all their weapons for overwatch? why?
Wife: "How much do you have invested in these ... miniatures?"
Me: "I don't think invested is the right word."