Yes it is possible to use a small vehicle. Carefully. The Northern/ 20 mile road has a beautiful surface until mile 5,5. From 5.5-9 you wonder if you will break the car there are so many holes. But you can make it. Things get better then even on Seboomook Rd but there are always places you have to be careful. Plan on two hours to get to Pittston Farm from Greenville. I know its only 40 miles.
Blackfly season is about from the 2-4 weekend to Canada Day.. Blame the Canadians for the BF. Then they are done. Only in failure ie very dry years do the little boogers try to have sex until September. Skeeters yes there are a few.
But nets not required. Just spray. The moose are also beset by skeeters which pretty much guarantees you will see a few on the river. Never not seen them on the river.
If coming from Millinocket you use the Golden Road which has more traffic and is usually in good shape though we have hit areas of deep sand for a stretch. The chief danger there is if they have recently graded and stoned the road. Shale is sharp. I don't see them doing that much after mud season ends in late May.
The cut off to Pittston Farm from Golden Road is the Seboomook Road.. Its got some areas where there are rocks in the road to avoid. Immovable rocks. Sometimes painted red just as a help.
Pittston Farm also serves dinner.. at least it did last year and the ones before.
some pics to entice
https://canoethewild.com/penobscot-river-west-branch/