If I write a word with a letter that isn't used, will I confuse my reader?

Background

So I recently had my first ever issue with the spelling of places in my novel. Someone pronounced a rather illustrious city, Ethil, incorrectly. Ethil is not pronounced with a 'f' sound as in 'fill', it is pronounced with a 'th' sound as in 'the' or 'þorn'.

To be honest, removing þ from the alphabet was a total mistake. It is an absolutely necessary letter and I am deeply upset that such a useful letter fell out of use. 'Th' has about 500 pronunciations anyway, so þ would be a good letter to help differentiate between them.

Question

Should I write Ethil or Eþil?

If I campaign hard enough or start using þ in my everyday life, will þ - the most useful letter - once again become a letter?

I used my own story as an example, however, this encompasses anybody's story where they might want to use weird letters, like æ for example.

So:

Will the average reader know how to pronounce these letters? How can you help a reader know which sound a combination of letters or letter makes if it has a lot of pronunciations, like 'th'?