Having not followed this thread up till now, and not having the patience to read 80+ posts, pardon me if I’m repeating things that have already been stated.
It is permissible to attend a simcha (between 2 Jews only, intermarriage is assur, period, end of story) in a non-Orthodox venue. I’ve called up many caterers after being told by the non-frum hosts that “it’s kosher” to ask whose supervision they’re under. And if it’s not acceptable, I can eat before or after, and just have a coke while there. Avoid the synagogue’s sanctuary, the rest of the building isn’t assur. As a family of many yeshivish cousins, we wait in the lobby while “services” are going on, and then join them in the simcha room. Obviously, if it’s on a Shabbos, skip it all together with a polite “I don’t want to ruin your fun with my restrictions”. That statement works like a charm!
Now, back to the original question about marrying kids of BT’s – I think that’s very neighborhood specific. There are communities like Passaic, Far Rockaway and Kew Gardens Hills that have a true mix of BT’s and FFB’s, and no one really knows who falls into which camp unless they know the people more intimately. And B”H, there are plenty of marriages between both. So perhaps you might want to try to hook up with shadduchins outside of Brooklyn.