Insane Byler proof #tome is Will going from trying to fix Mike and El’s relationship and sacrificing his feelings to keep them together to him looking for signs that his feelings might be reciprocated and even flirting.
Season 4 established that Will would do anything as long as it meant that Mike was happy, and this unfortunately included him selflessly passing off his own feelings as El’s because that is what he thought Mike needed.
He never tried to come in between their relationship (even though he had the perfect opportunity to what with the amount of times Mike kept telling him that their fight was “something you can’t come back from”). So it just wouldn’t make sense for the writers to suddenly turn around and have him become a homewrecker.
Unless. Unless something happened in those 18 months. The scene he had with Robin in the hospital only solidifies this more, because Will asking what defines “obvious” signals insinuates that Mike clearly had been doing something that was sending Will those signals. Enough to make Will genuinely question reciprocation, whereas in season 4 that was pretty much completely out of the question to him.
Will was recontextualising his interactions with Mike, and I believe that is exactly what the writers/producers will show us later on when Mike finds out the truth— old scenes gaining new meaning.
Either way I don’t think Will would be outright flirting with Mike if he believed that Mike and El were still going strong as a couple. As in Will wouldn’t suddenly become hopeful on his own, Mike had to be out here doing something.