With regard to attend the Bar Mitzvahs, no particular offense should be taken as we Catholic kids were not allowed to attend ANY non-Catholic religious service. I once lingered momentarily outside a protestant church when the windows were open and felt a little guilty.
When I was a kid back in the 50s, I belonged to a Boy Scout troop sponsored by a Presbyterian Church. Once a year they had 'Scout Sunday' and invited all the troop members to attend. I asked my parish priest if it was ok if I went. He said to the effect that I'd better go because that church did a lot for the kids and I owed them that respect, and BTW, say hello to Rev. Anderson while I was there. They were good friends.
I think that not attending another service was a myth that a lot of people believed, and not actually a church rule.