How would i say "you must not worry"?
I would guess "Rhaid i chi beidio â boeni."
Close - Rhaid i chi beidio â phoeni
Does the phoeni here come from a case of 'mixed mutation'?
Not mixed, just aspirate mutation after â (p t c > ph th ch).
The same aspirate mutation is found after ei "her" and after a "and", e.g. te a choffi, ei phwrs, gaeth hi ei thorri.
No, no mutation if you leave out the â.
I gave "there is no need to worry" as an answer - how would this be different?
Throwing out an answer although I'm not sure, it might be: 'Does dim rhaid i boeni', since the given answer word-for-word means "There is no need for you to worry."