I'm not sure why farang believe they're going to get answers from officialdom for something announced by the government that may or may not take place on X date, months from now.
There was a news release about this, but until it actually happens, no one will be able to answer any questions about it.
If it does get implemented on May 1, expect some instructions on what website to access and how to fill out the form, maybe 15 days before it goes live, not anytime soon.
2. No. 3. Up to you. You can apply for one online, at the border or in person at the embassy in Bangkok or consulate in Khon Kaen. Not many people who aren't traveling to a remote border (where visa on arrival facilities aren't available) and who don't have a car would apply for a visa in advance, though it is an option.
For entry via the Friendship Bridge, there's little point in doing so. However, an e-visa can help you save time at the border and space in your passport.
You have 3 weeks to go before entering Vietnam; that's not tight at all. The visa is the only thing you need to secure at least 3 business days before entering the country, unless you get an emergency 24 hour processing visa-on-arrival.
People often use these terms interchangeably. As long as the rest of the context indicates which one is desired, it's easy to provide the correct advice.