"Anybody know whether say Astros can sign Bourn & then immediately trade him to Mets for cash (assuming Bourn would agree to trade)?"
I think technically there is no rule against it as long as Bourn is OK with it and gives them permission to trade him, but then again all trades have to be reviewed by the commissioners office so that could be hold up.
"Any deal for cash only is highly scrutinized by MLB. They don't like players being exchanged for cash only."
It could be a trade for a prospect instead.
"Pardon my ignorance here -- How come Indians only have to give up 3rd rd pick but Astros have to give up 2nd rd pick?"
Both have their first round picks protected, Indians already gave up their 2nd round pick to sign Swisher.
So Mets trade them a prospect they would value equivalent to a 2nd round pick, They would only lose their third round pick so that is an upgrade for them, and the Mets would lose that player worth a second round pick but keep their first round pick so they also win.