#effect duration

[magicjudge]

If my opponent has exiled my creatures with Constricting sliver and I cast Overwhelming Splendor, what happens? If the slivers die do my cards stay exiles because there is no text saying they return?

You’ll still get your creatures back when the Slivers die. 

The ability that returns the creatures to the battlefield is part of the original trigger granted by Constricting Sliver and doesn’t depend on the Sliver still having the ability when it dies.

[magicjudge]

On the new card Avacynian Missionaries, what happens if I transform them through the normal method, exile a creature then transform them back using moonmist and sacrifice them, does the exiled creature come back?

Yes, it’s still the same permanent, so the duration of exile will end when it leaves the battlefield.

It’s better to just keep it around, re-equip it, and let it transform again and exile a second creature.