Im under the impression that if you buy the game through steam you must make all further purchases through the steam store. So probly no on the mogstation question.

As for subscriptions, you can also buy game time cards from several stores(including amazon) to get subscription time without ever setting up recurring payments.

And i believe they treat the 30 days as standard time, not entry time. Im not positive though, because i didnt personally see the point in making a second character in a game where one character can do almost everything.