They will let you back in 10 min later if you want, no problem, except in Chiang Saen and Chiang Khong. Be careful getting your info from clueless people.
Stupidity is incurable. You are 100% right about everything you said. I've dealt with hundreds of dog owners who abandoned their pets in Thailand (half are foreigners btw). Shelters are overcrowded and dogs are dumped there in jail.
All within 30min to 1h if you want. Depending how good timing is.
If you go to Vientiane ofc longer. The Train goes to Vientiane. So if you want the train back to BKK you have to take the train to Vientiane. I'm assuming the train waits for you guys at the border? Or you buy a train ticket at the border and wait for the train to come in the opposite direction? I'm not sure how it works it's very new.