the tethering feature is there in the settings... i haven't tried since i've been running on wifi only (waiting far a sim adapter so can use my microSIM from my iphone)... will know in a few days
Of course it is. The phone is 100% unlocked uncorrupted by any carrier. You just put your SIM in it. Tethering is baked into the settings menu. What you carrier decides to do if they find out you're tethering is up to them.
All of the accessories are the exact same. They're all i9250 models there is nothing different with them. Even cases for the Verizon/Sprint model will usually work because the sizes are so close. Verizon/Sprint batteries will NOT work. They are different sizes.
I would recommend you use the phone first then decide whether or not you want the extended battery. The official Samsung extended battery is only ~14% larger than stock.