The Conservative movement has been on the wane for decades. Just like the period when Shuls were taking over abandoned churches, it makes sense for Orthodox minyanim to use Conservative shul property (basement, social hall, etc…) for Orthodox minyanim, and eventually buy out the building itself, creating a new Orthodox shul. For this purpose, the edict not to enter a Conservative (or Reform, for that matter) shul is irrelevant and certainly does not apply. To claim that by doing this the Orthodox contingent would be tempted to join the Conservative movement is laughable.
I see here that this process is actually happening in certain communities, and it is perfectly acceptable.
On another level, attending a celebratory event in a social hall of a Conservative shul does not in any way lend legitimacy to the Conservative Movement.