Let's respond to your points one at a time.
Effigies.Whether it was only 6 people hanging the effigies pity nobody thought it a good idea to stop them as the fans might have got a bit of credit.
IRA songs.Hangover from the past eh,it's only because the situation in Northern Ireland is more peaceful currently that this is less of a issue as you can be sure as fuck the chimps would be back to singing the old shit if the situation there takes a turn for the worse.Has the hangover moved on now to displaying Palestinian flags( complete and utter arse holes).
Player getting abused.You must have had headphones on at the time mate,watched the game and the booing was by a sizeable section of the crowd.The next game at pittodrie was even worse as it was most of the support that joined in(I know cos I was at the game).
Attendances.Maybe you need to check your figures,the average attendances from the early 70s to mid 90s were between low and high 20,000s then suddenly jumped to high 50s,miraculous increase eh!
The Lee Rigby point is not about whether it was only a few involved its more symbolic of a support that has a unhealthy hatred of the armed forces of this country.