I've use this border probably 20 times since 2010. The last time around 2 months ago.
1) Most people can get visa at the border, no need electronic visa application in advance.
2) No need car in his situation. He just want to in and out quickly. The bus over the bridge is like $1.
3) Yes, I can answer that agent is never beneficial at that border. Whether it's to activate a Non-O multi entry, visa exempt, etc, no need for an agent.
4) No need to stay the night or to show any proof of. Very easy hassle free border.
In general, I always advice people to not use google as a reference for questions like these. Real life experience is much more reliable.
"How easy is to make it alone without any agent ?"
Very easy
"Is it really prohibited to walk or use your car at the Thai–Lao Friendship Bridge No1 and cross the bridge to Laos ?"
Car need paperwork, not worth it. You can walk if you like hour+ walk, but the Bus is like a dollar to the other side and take 5 min.
Can i simply pass the Laos immigration check point on arrival, then turn around, exit Laos and enter Thailand again? Do they accept it ?
Yes, you can. You get your Laos visa on arrival at the booth there and then turn around the corner 30 meters away and can leave Laos, retake the bus over the bridge, and re-enter Thailand.
(I did not wish to stay or visit any place in Laos at the moment)
Thai citizens can enter Laos only with Thai ID card or passport is required?
Nobody can tell by looking at ur stamp if u use an agent or not since it's the same stamp. So a real/legit stamp can be legally OR illegally obtained. Choosing an agent that can deliver is the main thing that matters. You pay a premium price to bypass financial requirements. But some agents are fake/scammer and run away with ur money or give u a fake stamp not even done by IO then u end up on overstay. So the risk is mostly when choosing an agent. It's especially first timer naive people that often get scammed. They trust agents with 10 fake reviews and weird ways of communicating.
If you buy a lotto ticket in Thailand on the black market (illegal) and you win, u will get paid (legit). The opposite would be a scam. You pay an agent or buy a ticket and u don't get paid or the agent runs with ur money.
It's called bribery. Illegal but not enforced. Agent, IO, and others down the line each take a cut. It's a common practice in Thailand in many fields. IO leaving is not a problem. Once u get the stamp you are legit. It's the same stamp as everyone else who doesn't use an agent. The IO put a real stamp, he just take a cut allowing you to go around the law. It's complicated to explain if u are not familiar with Thailand's way of doing things
The difference in price should be massive. If it's not then the risk is the exact same. If it's 5k baht ish and he just needs a babysitter then he has nothing to worry about.
I haven't try BKK entry since 2016. I'm not messing around with that airport. +, we don't know if the work permit he usually got was with an agent or not.