* Does BOS have a separate F lounge or shared with other classes?
There is a separate F dining section within the BA lounge (or there was, sorry haven't been to BOS since covid). Strictly speaking, your wife would not be able to guest you (unless she is GGL).

* Upon arrival at LHR, would my wife's F be able to get me into the Arrival lounge Concorde room or is that for F pax only (I don't have any status with BA or OneWorld; I'm a *A guy myself, very new to OneWorld)
You can both access the arrivals lounge and she can access the concorde breakfast area. Again, strictly speaking your wife would not be able to guest you (unless she is GGL). If you are connecting then she can guest you into the CCR airside at LHR.

* I could be wrong but I think I read somewhere that there are fast-track lanes for F pax at LHR, does that apply to J as well?
Yes, there is fast-track security for both but you would only use this if you are connecting. If you choose to go landside, your wife would be able to guest you to the FIRST wing as her ticket would enable you both to access GF and the CCR.

Best last-minute F award availability out of the US at the moment appears to be IAD or ATL.
The F cabins are chokka. Mixture of lots of people with re-booked flights, vouchers being used, money saved up and there were some excellent F deals to bag for 2022 just before the US officially re-opened.
Fingers crossed for you.