1

you can be there any time,, but some times heavy snow may cause trouble.. being a westerner you need restricted area permission, we INDIAN also need inner line permission to visit ARUNACHAL PRADESH, in kolkata it can be obtained from ARUNACHAL BHABAN IN SALTLAKE..
good luck.. !!

2

in kolkata is a arunachal pradesh tourist office
Assam Tourist Information Centre
8, Russell Street,
Kolkata - 700071
Phone : 033-2229-5094

Arunachal Pradesh Information Centre
Block CE-109, Sector-I, Salt Lake
Kolkata-700064
Phone : 033-2321-3627

3

Pretty sure to get the permit for AP you need to have 2 people...if done as a trip with a tour company/guide they can get a permit for one person, but if doing it in person I doubt you would get it as one.
Best time to visit is any time between now and April...after that it is getting too wet....it gets cold at night in places like Ziro and Raga so take a good hoodie/jumper....note: Dec-Jan is not a good time if you wanna visit Tawang as there is often snowfalls on the road and traffic delays (also damn cold then in Tawang).
You should try and time it for some festivals of the different tribes, which happen regularly....do some research....some festivals are Adi Mopin festival beginning of April, Apatani have one festival late March....plus many others..

4

Maybe now isn't the best time to visit, china has declared their intention to invade this province on their new passport.

5

I'll travel with my girlfriend so it's two of us. Can we get a permit (for westerners) in Gwahati in Assam, maybe in some travel agency? But we need only a permit not a full travel package.

6

pretty sure you have to get the permit in Kolkata....

7

Permits ( USD 50 ) is two persons minimum since a couple of years , 30 days thru approved travel agents . See arunachal tourism website for more details.

8

that's $50 per person.....not just for the whole permit.