I don't think RE and Citizenship are that important- I'm not doing GCSE Citizenship and we only did a Short Course in RE. I suppose it increases your understanding of world religions and moral issues and stuff, but it's not nearly as important as ENglish, Sciences, Maths IMO
It depends what you got 73/100 on- the paper or coursework? What % of your final grade is the 73/100? I would say that yes it's still possible to get an A whatever the % though, because I think an A is about 75% so you're not far off.