In the sentence:
What does こした mean? Is this 越す or 鼓する or something other?
And does まあいいや mean "well" or "whatever"?
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For the first part, こした is the past tense / completed aspect of the verb 越【こ】す. In your sample text, this is part of a set construction: ～[する]に越【こ】したことはない.
Some dictionary entries:
For the second part, yes, まあ basically means "well,..." and いいや parses out as basically "I guess it's good". As a whole, "well, yeah, okay then..."