Your math is correct. For the case of 'order does matter', you would indeed need to start decreasing the number of right runes, so the 2.98 · 10^(-30) probability is the correct value.

For the question of 'writing it in a fraction as the 1 in x chance', I believe that is accurate as long as the same full combination has the possibility to be reused at multiple different portals. Otherwise, *technically* you run into changing probabilities there because once a player finds a correct combination at one portal they know that combination can't be used elsewhere. In that case, the actual value of x in "1 in x" doesn't change by much in a practical sense, but it does change.