Is support for abortion a mortal sin? Euthanasia? Others?
With knowledge that it is wrong and you do it anyway, yes. The major infractions are things like stealing, lying, murder, etc., with knowledge of what you are doing and intent to do harm. There's a pretty good list which includes immodest dress (coug, cough considering what I saw at Mass on Sunday), not going to Mass on Sunday, impure thoughts, impure acts, etc.
Is there any moral/ethical difference between condoning an action and advocating an action?
Absolutely! Hopefully you are not jesting.