Can I extend a 30-day stamp from the Savannakhet border or should I apply for a tourist visa instead?

And IF you do get denied a tourist visa in Savannakhet and don't have two 30 day visa exempt entries by land yet this year you COULD get one entering at Mukdahan AND you could extend that for 30 more days at the immigration office for 1900baht.

Okay you were denied a tourist visa in Vientiane and you're out nothing by trying to apply for the visa in Savannakhet.

BUT

keep in mind you need proof of funds (20K baht or the equivalent in a bank account) AND proof of onward travel in 60 - 90 days to apply for a tourist visa at the thai consulate in Savannakhet.

As far as will you get in on a 30 day visa exempt entry at ANY border with lao? That totally depends on IF you have TWO 30 day visa exempt entries by land so far this year. That's because you're only allowed TWO 30 day visa exempt entries in a calendar year (Jan 1 - Dec 31).

So my advice to you would be IF you can meet the requirements to apply for a tourist visa in Savannakhet try it.

And for the second part of your question, I answered that already in your first question.

you can only get a 30 day Visa Exempt Entry stamp twice a calendar year if you enter by land borders, as i sit relaxed in my arm chair and have no access to your passport please check yourself.