In my opinion, the MTA would be better off giving Gillig a shot (assuming that Gillig themselves would be interested in doing so, which is far from guaranteed) than going back to Proterra. It is very telling that Proterra did not receive any orders from the authority after participating in the electric bus test, while NFI did. Let other TA's be the guinea pigs while Proterra works through the various issues with their buses and tries to build up the capability to produce large orders with a high degree of quality.
I do think that the days of the MTA throwing its weight around with bus manufacturers and getting multiple companies to do their bidding are over. Both Orion and Nova seemed to be hoping to be the authority's top bus manufacturer, but their European parent companies pulled the plug when the MTA/NY governor's office showed no favoritism for bidders with a factory located in the state (I'm not saying that they should have show favoritism, but I wouldn't be surprised if that mindset played a role at Daimler and/or Volvo). New Flyer is the last big manufacturer standing unless Gillig throws their hat into the ring, and the MTA frankly needs NFI more than NFI needs the MTA. Even if Gillig does decide to get involved with the MTA, I think it would be very much on their terms - their management has been conservative over the years and would almost certainly want to protect the company from getting entangled in a messy and expensive situation with the authority. A replay of the 2011 bus evaluation program would likely be ideal - say, 90 to 100 buses that could be assigned to a few depots to see how the Low Floor model performs in its most intensive test yet. I also wouldn't be surprised if the MTA reaches out to their counterparts at NICE to check on how their Gilligs are performing.
Just my 2 cents worth.