FINDBACK() is a little awkward to implement without regex in a *plain* formula, but not impossible.

Here’s how the final formula is built up, exploded in digestable steps:

First find how many non-spaces there are:

=LEN(SUBSTITUTE(A1," “,”"))

Find how many spaces there are:

=LEN(A1)-LEN(SUBSTITUTE(A1," “,”"))

Replace the space at the [number of spaces]-th occurrence with a marker (choose an unused character):

=SUBSTITUTE(A1," “,“☿”,(LEN(A1)-LEN(SUBSTITUTE(A1,” “,”"))))

Find the index of the marker:

=FIND(“☿”,SUBSTITUTE(A1,“0”,“☿”,(LEN(A1)-LEN(SUBSTITUTE(A1,“0”,"")))))

Credits: https://stackoverflow.com/a/25820019/1052284 and yours truly

Note that this crashes with “Err:502” if there are no spaces. To cover such cases it gets even more unwieldy:

=IF(LEN(SUBSTITUTE(A1," “,”"))=LEN(A1),LEN(A1),FIND(“☿”,SUBSTITUTE(A1," “,“☿”,(LEN(A1)-LEN(SUBSTITUTE(A1,” “,”"))))))