Does a LET variable get computed even if it's only referenced inside IFNA's fallback argument?

Without doing some testing I'm not sure but I do know that certain functions will calculate fully regardless.  So I do NOT think this is a function of the LET but rather a function of the IFNA and I haven't looked into the IFNA but the I know that the IF statement will NOT calculate the unused case in 'normal' or basic conditions but WILL calculate fully when it is passed an array for the conditional.  So in your case, I think it depends if that conditional is or is considered an array then I suspect it will fully calculate.  Basically (at least in the case of the IF statement) it goes from a native operator to a function with parameters and excel will calculate those parameters before passing it to the function.  That all said there is a bit of work done using LAMBDA functions as THUNKS to get excel to pass those functions as unresolved functions and then only calculate the result at the end and showing significant performance improvements doing it that way.  I'm not saying it will solve your issue but may make it more efficient if you are in need of performance improvements.