Wedding Shower Thank You Etiquette

I don't think an apology isn't necessary. Most people are just happy when they get thank you cards.

No, not at all. 4-5 weeks isn't that long of a time span.

From what I've seen/read, within 3 months is acceptable for a Thank You card. You're still within that time frame, so you should be good

No need to apologize, just the gesture itself is always appreciated.

I don't think its necessary to apologize, just tell your mom and FMIL and they can share the word if anyone makes negative comments.