Myanmar had a Coup d'état/genocide going on since last year, so the border tourist in CM usually use for border run won't be open for them anytime soon. It's way too dangerous of a country to border run from.
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.