my original thread got locked and there's misinformation that the official answer doesn't clarify. I'm put this up again so other users aren't lead astray. I wanted to put this in the Tips forum, but I can't select in the drop down for some reason.
The help article linked to me to solve my issue was
this article discusses updating existing MIB files.
My issue is that we could not get the MIB files for different devices to load in the first place, so there was nothing to update. Moreover, that article focuses on the GUID-base, which were all unique for the different versions we were trying to post. We were following all the recommendations in that article.
The missing information is that you need to have the version numbers be hirer than previous uploads across all device versions. No where in that article is that specifically addressed.
So for example, if you uploaded the 7th export of your iPad-specific MIB it would have .7 in the suffix. If you then wanted to put the second export of your Android phone MIB (thus with a .2 suffix), you'd get this error even though the GUID-bases are different and the devices are different.
You need to manually change the suffix number to be hirer than the previous number regardless of what the GUID-base is or what device you're trying to post for. i.e., your Android phone version would need to be .8 in the example above to get it to load.