You can extend on the 18 with no overstay penalty because the 16 is on a Saturday, but as Stuart said the 30 days is added to the end of your Visa so 16th of April + 30 days, regardless of which day you go extending.
can tell you if I'm making things up (which is possible), but from everything that I remember about the Non-O extension based on marriage, the financial requirement is only if a foreign man marry a Thai woman, not the opposite.
Yes, it needs to be in your account 2 months before applying for the extension based on marriage. Its not money taken away from you it just need to be sitting there. It just act as a proof that you can support your wife for that full year if needed. If you marry a Thai man I dont think that you have that financial requirement. Same sex marriage is illegal in Thailand.