Many heart pigs sound fine on stethoscope. I had one sow, Tuppence, I am absolutely sure she was a heart pig. She has so many of the classic symptoms, but because she sounded fine on steth at both a vet and CCT, I could not get the help for her.
My second heart pig, I had niggling suspicions prior to her actual diagnosis but was mollified by hearing it was ok, but the suspicions surfaced when new symptoms developed and - thankfully, yes thankfully - CCT picked up on a muffled sounding heart (fluid around the heart). Gwenny's response to the diuretics (initial improvement, but fluid returning despite daily doses) provided the basic diagnosis that she was a heart pig.
I know only too well the frustration and difficulty of suspecting something and not being listened to.
For now though, with you and Basil, have to think short term as in the here and now as this is what is most important, and trying to get him over this relapse in condition, and for me I still think diuretic and antibiotics is the way to go.