Vast majority of embassies require the 500k in a regular bank account, You would need to contact the embassy you apply from if they approve of your retirement account being used
yeah the initial requirements are valid throughout the visa validity and why i chose Workcation. I was told by the Thai staff who interviewed me to do border hops "you won't need to show anything, it is simple. If you go to immigration it will be too much work". I keep my DTV docs on my phone, just
You should check your local Thai Embassy website for the application process. There are SP courses with Thai language as part of the course. Full time Thai language comes under Education visa so it is not really issued via DTV. If you extend your DTV at immigration you need to show course renewal docs if the course has ended. No current requirement to do so with a border hop. How long it takes depends on where you apply.