Yes anything you have found online is no longer applicable. Everything is done online now. You can apply online as soon as you arrive in Vietnam. But as Stuart said, no one knows what the processing time will be. It varies greatly from embassy to embassy. For example in London they process most in 2-3 days. In Paris they take 1-2 months. It's anyone's guess at this point.
that part will be up the amphur. Each one has their own requirements. Some are trivially easy while some are actually impossible and won't do it at all. There are hundreds or thousands of amphur across Thailand and each gets to make their own rules
Someone else posted a couple months ago that the Chiang Rai immigration office refused to let them even apply for the marriage visa after they had all the requirements. Impossible to know the full story, but you might want to look at alternatives.
I don't know where you get your information, but e-visa is the same in every Thai embassy in the world that has adopted the system. It is ALL online ONLY. You ARE NOT ALLOWED to go to the embassy. Only Thai citizens are allowed in the embassy after e-visa is adopted. The embassy even posted about it and clearly state that it is all done online. You need to get better sources of information.