1) If we have asked some relative earlier and they told they also always buy their meat from muslim supermarket and it is halal.

Hence I assumed it was halal and ate the next time without asking at first. Them I asked them again and one of them told it was from non muslim store at first but then another person told again it was from muslim supermarket and that she hade bought it from there. Was it halal for me to eat? (because I become doubtful)

2) will my prayers be rejected the next 40 days?

1) You should have not eaten it, as long you are in doubt.

2) No