sorry forgot 2nd q
the approx cut off date for Manx Museum is c.1916 - before this date it is likely the Manx Museum has the copy - after this use the registry. For wills the registry needs advance notice so if you need post 1916 wills email them.
For any official certs you need General Registry + pay for each cert (or to check the index)
before 1877/8 there was no official data collection and all you have are the baptismal /mariage/burial records of the various churches (which had however to be made available to registry) and these are on film at Manx Museum who also have most of the original registers - Roman Catholic records were not generally released to the Mormons (tho recently they have obtained them via PROs) - 19th C RC records are on film at Manx Museum