You are confusing things. Borders have been open for a long time, but mostly for trucks, locals, etc, not for tourists. For example, the border in Nong Khai have been open for more then 1 year.
The ironic thing is that you are very firm about what you want this thread to be yet you are asking one of the most asked question ever on this thread. You can type keywords and find the same question everywhere in this group. To answer your question... the border is only open to people with a Laotian guide, so very few people have gone there. And those who did have not come back with a tourist visa. I've read only 2 people who did the trip and they had re-entry, they were on long term visa.
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.