The "ə" is a Mid-central vowel, which does sound similar to a "u" (close back rounded vowel), but from a linguistic standpoint they are considered different. I am sure it depends where in Ireland you are getting the pronunciation from too. "ə": https://en.wikipedia.org/wiki/Mid-central_vowel "u": https://en.wikipedia.org/wiki/Close_back_rounded_vowel IPA Vowel Table: https://en.wikipedia.org/wiki/Template:IPA_chart/table_vowels
Lenition and eclipsis have to be caused by something.
maith is an adjective in maidin mhaith, so it agrees with the gender of the noun that it qualifies - maidin is a feminine noun so it's adjective is lenited.
There is nothing in Go raibh maith agat that would cause maith to be lenited.