When performing a song publicly, it is advisable to pronounce the lyrics as the author may have done it, not necessarily "correctly" by linguistic standards. We hat this discussion e.g. about "Oh Rio!"

The song in question refers to the opera by Jules Massenet, based on the novel by Anatol France, thus a French pronunciation should be most adequate (i.e. T rather than Th). However, the usage in the verse metre of the song is such that of the two syllables of the name, the first one has the stress, so it's "anglicized", and you may as well pronounce the Th as you would in English.

For those who don't know this brilliant song and its author, see Wikipedia.

As for what characters Mudcat will "allow", i.e. store and reproduce faithfully, best rely only on the basic modern English alphabet. This problem is another long-runner, and the "solution" now in force seems less satisfactory to me than what we had decades ago. Anyway, for all other characters use a tool to produce HTML escapes, as described by the above posters. (I think I offered several such tools written by myself, which is not really difficult. You can ask ChatGPT to write one for you.)