I get that eminent domain is a touchy subject, and I'll admit that my biggest character flaw is easily my propensity towards tearing down or punching through things I see as obstacles to progress. It's why I'm so forward with advocating for eminent domain takeovers (among other things), something that I've been assured is quite terrifying to other people. And at the end of the day, taking the scorched-earth stance probably does more harm overall than it does good - but, I think the same can be said of going too far in the other direction, of being far too gun-shy and so unwilling to take an unfavorable position that you end up causing your projects and yourself far more trouble than you would have if you'd taken the unfavorable position to begin with.
Well, to move back towards the original topic - I could easily see a Westbrook Station getting built in the vicinity of Cumberland Mills, probably off of what looks like an
empty lot on Warren Avenue. I think you're selling yourself short by pushing for a flag stop - the geometry of the curve introduced by the Westbrook detour is far too unforgiving to be cleared at a rate of speed faster than 10 mph. Once you decelerate that much, it hurts you nothing (or else close enough to nothing to be a rounding error) to come to a full stop. Back Bay is an excellent example of this in action.
I don't think picking up Westbrook is enough to take the sting off of the Westbrook detour, and it certainly wouldn't be reason enough on its own to go forward with this - but it's something to think about. Also, to the point of the credit union - I'm not familiar with that credit union in particular, but as I understand it, most credit unions tend to be favorable towards community development projects. I think there would exist the opportunity to come to a favorable agreement with them for the land, especially if you can sell them on the merits of passenger/commuter rail in the area and then give them space in the station itself. Again, I don't know if a proposal along that line is actionable, but it's probably worth some consideration at least. Certainly, I'd expect the McDonald's and the Subway that the Westbrook detour would end up demolishing to move into PTC and/or a Westbrook Station, and probably at a reduced rate as part of the buy-out package.
There are options for taking over the land that aren't totally negative - just because I'm not particularly averse to exercising eminent domain powers if and when negotiations break down doesn't mean I wouldn't much rather come to a mutually beneficial agreement with all affected parties. After all, "New Westbrook routing of the Downeaster moves forward with full support from local partners" sounds a whole hell of a lot better than "After deal for land acquisition falls through, local businesses prepared for lengthy eminent domain battle," does it not?