To add to the new reasonings in the OP, I think the justifications for the likely and definite low multiverse level ratings should be changed. I would like to propose something for Son Goku UI, Jiren BUW, Gogeta SSJB and Broly FPSSJ
Son Goku (Ultra Instinct): Low Multiverse level (Completely outmatched Full Power Jiren, who was stated to possess abilities greater than the Gods of Destruction)
Jiren (Burning Ultimate Warrior): Low Multiverse level (Fought on par with Ultra Instinct Goku)
Gogeta (Super Saiyan Blue): Universe level+ (Completely outmatched Full Power Super Saiyan Broly), likely Low Multiverse level (Son Goku stated that Full Power Super Saiyan Broly is “probably” stronger than Beerus)
Broly (Full Power Super Saiyan): Universe level+ (Superior to Super Saiyan Gogeta), likely higher (Stated by Son Goku to “probably” be stronger than Beerus)
Tbh I don’t think Broly should get a likely 2-C rating, because he was only said to probably be stronger than Beerus, which doesn’t imply a stomp like the other characters mentioned above. I think for Broly, it should be “Universe level+, likely higher” higher referring to “At least Universe level+”. And since the “probably” was used for an At least Low 2-C for Broly and it wasn’t used correctly, Gogeta would preferably be labeled as “Universe level+, likely Low Multiverse level” Gogeta wouldn’t have the At least before Low 2-C anymore, because his Super Saiyan Blue form doesn’t have any statements we know of regarding its superiority over GoDs like Jiren
Edit: I don’t see why UIO3 Son Goku and Full Power Jiren would be given a likely 2-C, so I didn’t touch on the subject