It would be easier to come to Thailand on a tourist visa and start the process over. Need a lawyer to make sure you don't get screwed over while here. Thailand have had his fair share of love scam and internet scam from Nigeria and other African countries therefore they often are very paranoid about allowing people from Africa to come here. Being here and involved in the process directly with lawyer, and access to the immigration office, etc, would help a lot.
You are black, and African. These are the reason it's taken so long. If you were white from the UK, you would be okay already. Yes, they are taking longer because of that and the chance of rejection is higher. No, I do not think that's it's fair, but it's the world we live in.
It doesn't matter where you get what. You need to be stamped out of Thailand, and in Laos. You can't get back into Thailand if you are stuck between two countries.
You absolutely need to cross the bridge to border bounce/run. To enter Laos, you need your visa on arrival which is on the other side of the bridge. To go there, you need to take the 35 baht bus like everyone else. There is no way around it. Then, when you have the visa, you turn around and leave Laos. Then take the same 35 baht bus back over the bridge. Then, you enter Thailand with visa exempt entry.
Also, those dedicated line I've not seen that for a very long time. And this is part of what I said about not arriving there went group bus are also there.