Is תְּשׁ (a ת with shva followed by a שׁ) a digraph for the "tch" sound, as in swiTCH?
It's not a digraph, as the correct pronunciation is "Tishuva" (a very short i after the T).
The audio is how people usually say it.
Could you refer to a source about this correct pronunciation? Seems a little weird
The ת gets a דגש קל for being at the beginning of the word.
Therefore the שווא under it is נע.
Why should the שְׁוָא נָע (or Shva mobile) be pronounced [i]? In modern Hebrew it is an [e] or is omitted.
In loanwords צ' is usually used (צ with an apostrophe).
So it's "tch", unless it's a loanword and then it's spelled 'צ?
This is true both for European (צִ'יפְּס chips) and Semitic (צ'אדור chador) loans.