OK Paolo: all points in your above Post noted.
I cannot think of any further helpful comments on your BFO problem.
Re: the other / general comments on servicing this receiver: if you haven't already done so, there are a lot of entries on this Forum detailing repairs & measurements that I and others have done and the results so obtained. Put them all together - and you can get a lot of useful info.
Persevere with it - IMHO the Racal RA-17 (& its derivatives) is one of the finest valved comms. receivers ever built.
Al / Skywave.