3 attendances of below 5000 in the "second tier" would seem really odd these days, but the stand-out low crowd was the 10,263 at Chelsea. I have a programme from the season before, early on in the season, which lists average attendances for the first five or six games, and ours was above Newcastle's.
And Swansea were top of Div 1 when we beat them 2-0 at Oakwell, before losing 2-3 at Swansea in the 2nd leg (the first two rounds were two legs). We then won 4-1 against a Brighton side that had won at Tottenham a few days earlier, before beating Man City in front of ca. 33000 at Oakwell.
As you say, the FA Cup was huge then. Our replay against Enfield at White Hart Lane drew a bigger crowd than Spurs v Arsenal had done, a few days earlier. That sounds pretty incredible now.
I was there ,outnumbered by everyone in London cheering them on. We won , but my it was a quiet walk back to the car .
Garner always scored but I went to three wins at Ewood. I'd say it was about 50-50 between us results wise plus very similar grounds and attendances. Then Jack Walker bought them 20 years above their level.