Maybe this Travelogue will give you some ideas about driving to Arunachal! https://sites.google.com/site/hvkumar/kumar'snortheastdrivingholiday
North East is best in winter before April because there is least water - and hence river crossings & better roads - and after that it rains incessantly till at least September. The rains leave behind a trail of destruction of roads and it take still at least Dec for roads to be repaired. Places like Kibithu (eastern most) are not accessible except during winter. Wherever there are ferry crossings, there are significant differences in time for crossings across the Brahmaputra & Lohit Rivers. The main highways in Assam are excellent.
Roads from Siliguri to Bhutan border of Phuntsoling is excellent but if you are looking to go to Guwahati, both the main highways are in bad condition. Of the two, prefer the one via Cooch Behar & Gauripur, and not the one via Alipurduar & Bongaigaon.
Getting permits for Bhutan is no issue at all. You can obtain visit permits in Calcutta itself, but the Phuntsoling Immigration is faster and efficient, except that you have t present yourselves there during office working days/ timings. Moreover, you have to obtain your car permits at the RSTA Office there. Multiple permits for travelling beyond Thimphu & Paro to Central & East Bhutan, from the Thimphu Immigration & RSTA required.
To go into certain Dzongs, you need a Cultural Ministry permit also. For example, you can climb up to the Tiger's Nest in Paro Valley but cannot enter the Dzong/Gompa without a permit.
Your friend who did not get the permit must have been denied it because the border was closed for 1.5 months during the SAARC Summit during the September period. Make sure you do your research on such things before you travel. Bhutan Govt officials are super-friendly and you can call them up and check out these things before you plan/ travel. Please remember that Indians are welcome and need hardly any formalities to travel there unlike other foreigners who can travel only as part of conducted tours in pre-arranged taxis, and through tour operators. Only Indians can plan their own drives in Bhutan. I have driven there both by mobike (1994) and car (2009).
Road from Tezpur to Itanagar is also excellent.
There are no safety issues anywhere en route to Bhutan (or in Bhutan) and even in Assam, but for the frequent bandhs, an ordinary tourist faces no problems unless he tries to act "smart" . Keep tipping anyone who stops you on the road. Please respect local sentiments and do not get into ideological arguments on Maoism, ULFAism, Bodoism and such like, simply concentrate on your drive.
To travel anywhere in Arunachal, you need a permit which has to be obtained in advance at either Delhi, Calcutta, Guwahati, Tezpur or Dibrugarh. Withot this, you cannot cross check posts into AR.
Road to Tawang is good except during rainy season and in some sections around Se la. Remember, you need special army permits to go to Bum la, the China border beyond Tawang.
You can also read my drive account to Tawang in this 2001 travelogue: MOTORING HOLIDAY IN NORTH EAST INDIA
You can check out log books of all my drives to the region here:
1994 mobike ride to Darjeeling, Sikkhim & Bhutan https://sites.google.com/site/hvkuma...vinglogbooks18
2001 drive to Tawang & rest of North East India https://sites.google.com/site/hvkuma...vinglogbooks37
2009 drive to Sikkhim, Bhutan & Arunachal, including easternmost sectors kumar'sdrivinglogbooks46 - hvkumar