[Sashimister]

Originally Posted by pacerier

cool, just to check, if i were to say something along the lines of "Bob has a foul mouth", are both of these acceptable: ボブは口汚い / ボブは口が汚い

Both are good. Tell me in which one 汚い can be read in two ways.

if みだら is written in kanji, will it be 淫ら or 猥ら, or is it that actually we do not write it in kanji?
(will the 心 in みだらな心 pronounced as しん or こころ?)

You CAN write it in kanji.

心 all by itself is always read こころ. If you didn't know this, don't bother writing みだら in kanji yet. I don't think it's an effective method of learning Japanese if you don't have all the basics down but want to write words in kanji that many native speakers prefer writing in kana. みだら is a 大和言葉 to begin with as you should be able to tell from its sound. Kanji like 淫 and 猥 look too direct and obscene in our eyes.

Also, other than using 必勝法 as “sure-win method” for games, can we use it in the context of competitions / battles / wars etc? E.g. この試合、必勝法があります

No, not really if you mean a soccer match or baseball game. People may say it but theoretically it cannot exist, can it?