You are all good, no worries.
I think all the data is important. I would prefer to lose the performing artists in favor of the proper ISRC for the main reason that I prefer partial correct data rather than complete incorrect data. I of course an generalizing with that statement, but to combine 2 recordings that have different attributes is wrong to me.
For me, and this is just my personal opinion, I do not tag with performing artists or other more detailed credits like that. I am finding it best to tag with items like a UPC, ISRC, etc. If I have that info, I can then easily find the rest in other places. I believe that the most important factor in tagging is to properly identify the song/recording/etc, what ever you want to call it, that the file contains. For proper identification of performing artists, ISRC is a great tool, since a change in performing artists warrants a new ISRC. I am fairly new to using ISRC in such a way, but it was motivated by the many difficulties of tagging in addition to iTunes and other stores listing this for all the recordings sold in their service. So it is becoming a constant that all recordings tagged will have.
I do not tag any longer with Picard, but for tagging, I know you can select the release you want to tag from. So if the release you are tagging has 5 versions, you can select the one to use from the MB database, thus selecting the one you have filled out completely. A different release could very easily have different attributes, thus why it is a different release in the first place… like in this case, the credits relating for mastering, the acoustID, the ISRC, etc.
I believe it is the case for all recording with multiple versions… the extra relationships like performers have to be entered to each. Maybe the classical people can assist on how to handle this better as there is a lot of detail there. I think having the performer information within MB is great, and although I do not want it in my tags, it is a place I can get that info from the data I do tag.
Briefly, since it is sort of off topic but mentioned… I have struggled with tagging and settled on the idea that tagging with the common data of artist, title, album, album artist, date, UPC, etc…, having ISRCs for each track, etc. makes it easier. Reason is that it keeps the tag data smaller and it provides me with all I need to accurately identify the release and each of its recordings. So I can take my reference data and be able to pull any data I want for that release and its recordings, assuming I have a place to get such info… like MB. I am hoping that other large sources like iTunes, Spotify, etc improve on making data available as they are all now using ISRC and UPC as identifiers, which is helping create a common ground.