That depends entirely on how long you've been here this past 6-12 months on tourist visas or visa-exempt entries, and which immigration officer you end up in front of.
The more extensive your recent history, the greater the likelihood you'll be pulled aside and questioned about what you're doing here, and potentially denied.
When that point comes is where IO discretion comes in. Some people get questioned after just 2 or 3 such stays, others manage to live here for over a year before they run into problems.
Seems like the new E-Visa system was not even close to being ready for rollout. Seems to take a very long time compared to other countries, given all these cases of people applying way in advance of their flight, only for it to not be ready weeks and weeks later when it's time for their flights.
Back in 2018, I visited Burma and even their visa system was orders of magnitude faster; had mine within a few days.
I can't say for certain whether you'd need to buy a Lao visa again.
Usually Nong Khai is the easiest, most highly-recommended place for doing border bounces, for people who haven't used up their allowance of two in a calendar year.
Whether you get to re-enter depends ultimately on which imm officer you're in front of.
If this imm officer told you this *at the exit booth*, then you'll be dealing with someone different at the entry booth.
You *can* still try today. You may be denied, at which point you can try another entry point. They may also let you in, but after a long grilling, and emphasise your need to get a long-term visa or leave.
My mistake - I forgot that sometimes people need to transfer between airports and not just fly in at Suvarnabhumi and then straight out from the same airport.
In that case, however, I'd imagine that immigration would be happy with his onward ticket to India?
If you fly home via BKK, you don't actually *enter* Thailand. Since you're just transferring flights, you don't go through passport control.
Also, you absolutely *can* try to enter with your visa again at Nong Khai. If you're denied entry there, you still can *try* another entry point.
Whether you get denied when you come back to Nong Khai depends, basically, on which individual immigration officer you end up in front of.
As to the likelihood that said officer will question you about what you're doing here or potentially denying you entry, that depends on how long you've been here on back-to-back tourist visas & visa-exemptions.
The more you have, the greater the likelihood. Some people are able to do it for over a year or even two years, others get questioned after just two or three.