In brief, it's bribery. The bribery in Phetchaburi is not recognized in BKK, and vice-versa. Went you pay an agent good money to help you, the agent pay IO and more down the chain of command. Only a fraction actually stay in the agent pocket.
Nobody screwed you. Agent are all illegal. But your stamps are legit. Your stamps won't be recognize by BKK because of how you got said stamps elsewhere. Not a single agent could have prevented what happened to you today. The recommendation you got from CW IO today is what everyone does went they want to extend without using an agent they previously used. You need to leave the country like he said and apply again.
First of all, you are NOT on overstay. A lot of awful misinformation going on in the comments. What happened is that you used an agent last time, so you are bound to use the same agent + same immigration this time around too. It's the downside of using an agent, you are stuck with them for future extensions because they way you obtained said Non-O last time was illegally obtain but the stamps are legit. To erase that, you need to leave the country and apply for Non-O in neighbor country. So when you come back in u will be able to extend for a year no problem.
He is not on overstay. But because he used an agent a year ago, he is stuck in using them again with that same immigration. That's the downside of using an agent, unless you leave the country and come back with fresh stamps you have to re-use the agent forever.
Yes and No. First, please understand that EVERY agent are doing things illegally per Thai law went it come to helping you out for whatever visa or extension related, even in your case. Second, some agent are just there to scam you, they can't give you what they promise, but others (like Thai Visa Centre) will 100% provide what they promise you, and your stamp will be legit. Something can be illegal but legit. For example, the black market for lotto ticket is illegal in Thailand, but if you buy a lotto ticket from the black market and win, you will get paid, therefore it is legit. The opposite of legit would be a scam... you give 20k baht to an agent and he run with your money. Selecting an agent that is reputable matter, because the scammy one rarely if ever face consequences for scamming foreigners.
The whole point of using an agent is to get around the financial requirement. The agent will put the money in your account for a few days, or won't even, it depend.
He's not getting con (TVC) own this group. You can see their details on the banner above. They are legit agent, but you are correct that OP doesn't need that. But if he is last minute and doesn't want to bounce then he has to use them.
OP is confusing getting married with getting a marriage extension. You nearly have the OK to get married. Getting a Non-O then a yearly extension based on marriage is completely different from getting married. Getting married is the requirement to apply for the Non-O, it does not give you the visa right away. You are way too late to apply for everything unless you use an agent or bounce. As others have said, just do a border bounce and come back to us when you get your marriage certificate for more advice. It take 1 hour to get married, not 3-4 weeks. The process at the district office should not be weeks, I think you are confusing a lot of things therefore we can't advice you well.
Illegal in the sense that you cannot work in Thailand without a work permit or LTR. So you cant come here on visa exempt or tourist visa and work online until you get work visa or LTR. This is illegal. BUT... this is rarely enforced. Unless you talk about it online, nobody will know nor care about what you do at your house or in a random coffee shop.
"I run and will continue to run a ltd business in the UK and that's where the money will come from to live in thailand.". This is ILLEGAL but NOT ENFORCED. Which mean that you can still run your business online while in Thailand, but avoid making post like these where you tell the world about it! :)