For Mysterious Island #7, can the new coin be a UT or a 0 coin?
5-6: After exploring this island, place a treasure coin from your collection with value equal to or less than one you took on the island.
It specifically refers to a coin with a value, so 0-gold coins are allowed.
Most UTs would not count because they lack any gold value, although one that did have a gold value (determined when found/loaded) would be okay as long as that value was less than whatever other treasure you took to set the max value.