The TL;DR is in the second-to-last paragraph. When Union hits ten TUC games, their pairwise rank is going to fall. No ifs, ands, or buts. (Same thing with UML, by the way.) But, Union has the opportunity to make up their poor results so far by (in order of importance) a) performing really well in their remaining four TUC in-conference games, b) Rensselaer becoming a TUC, c) Merrimack becoming a TUC.
If that happens AND they get some breaks (SCSU faltering a little to drop their TUC Record under 0.500 / Yale finishing behind Union in the standings / no upset conference tournament winners), then Union can sneak their way into the tournament.
Once all of the Big 4 teams get up to 10+ TUC games (and everyone has played everyone else within their conference), these types of answers will be easier to divine. I don't have the bracketology expertise of Red Cloud, but I have some kind of numbers background, so I'll try and keep my eyes open for the ECAC teams; see if there isn't some way to make their path forward make a little more sense. The basic mantra is always cheer for your non-conference opponents (unless you performed epicly bad against them and they're just below the TUC Cliff).