So to check theory about whenever MIBAM causes issues to serial timings, I've connected 16 led strip (longest I had at moment) with APA102Cs to it, and mibam code was running in background and I was shifting out data without any issues.

its not a valid test, it means nothing. shiftout clock timing for a slave is synchronous , serin serout are asynchronous
pbp has no synchronous reception commands as a slave device