Some loose thoughts:

1. Depending on your follow on strategy you might want to bring a new bucket of capital vs deploying out of your existing funds. Other times you might have a deal that generates a lot of excitement from some of your LPs and less from others, the SPV gives the ones who love it an opportunity to increase their exposure without dragging everyone along for the ride.

2. Assuming you mean once its invested in a venture (startup) it's gone once the check clears, unless the startup returns capital to investors for some reason in the future (bad). If you meant once its in the fund, somewhat similar story. If you're talking about a non Start Fund, where capital is committed before its called, that's a different sort of dance and really comes down to managing the relationships with your LPs

3. I've really only seen European, especially for emerging funds. Its favorable to LPs, and they have all the leverage to make sure they get their money out first. I don't know of anywhere the structure is legally mandated.