First, I think hosting a USHL team would be a great move for Manch and its arena. No question about it.
I do want to at least try to disabuse you of the Monarchs' decisions to leave Manchester being "presumably for economic issues". Manchester was at least initially wildly successful as a new AHL city at the turn of the century in their brand-new building, playing to 80+% capacity in what was still a pretty big facility for a minor league team. Then the '08/'09 economic downturn, and loss of the "new car smell" set in, and figures fell. But even in their final 7 or so seasons as an AHL affiliate, they were no worse than middle of the pack attendance-wise across the AHL, having stabilized in the 5,500-5,600 range or thereabouts. As you can see from the figures on the attached links, that was also a pretty average figure for the AHL, and comfortably ahead of other Northeast/New England cities like Worcester, Portland, Springfield, Albany, Utica, Glens Falls and even Hartford (which still makes noises about getting their NHL team back someday lol):
AHL 2014-15 team attendance at hockeydb.com
Manchester Monarchs [AHL] yearly attendance at hockeydb.com
The Kings' decision after the Monarchs' 2015 Calder Cup run to swap their AHL and then-ECHL affiliates (Ontario Reign) resulted in Manchester getting saddled with goon league hockey for its final 4 years. The economic issues driving that swap were not attendance-related, but a desire of several NHL teams away from the East Coast/Northeast to have their top affiliates closer to the parent club's location ... and Manchester was just unlucky to be an affiliate of a West Coast franchise. I also suspect fans in and around ManchVegas were feeling a little disenfranchised by the departure of their Calder Cup champs, and attendance basically halved. No lipstick on this pig, attendance fell almost to the bottom of the AHL. But as someone who has experienced losing a team (entirely) in a different sport ... I get the negative vibe, I'm guessing the frequency of my trips to Montreal would have cratered too if they were given an International League/Eastern League team instead. Throw in ownership issues (I believe the original owners went to CA with their AHL license), and yes, it was all that AND declining attendance, etc.
ECHL 2018-19 team attendance at hockeydb.com
Concord Monitor - Plagued by declining attendance, Manchester Monarchs have played last game
When I hit the next billion-dollar lottery, one of the things I'm doing is buying myself a minor league team, I tellya ...