And as I said, all the same layout options are available for playlists as for the library. If it looks different to you it's because the settings are different. You can change that. The info you need is in the wiki.
You're talking about layout inside a current playlist. I'm talking about Thumbnail Browser presence.
You mean that i can add Thumbnail Browser and the Playlist Swicther both inside PLAYLIST tab?
Yes, you can go to the layout settings, next to the search box, and add or remove anything you want. You don't have to have a playlist switcher (without a screenshot, I'm not sure exactly which part that is but they can all be deactivated, one way or another). You can add thumbnail or column browser.
I'm willing to accept that what we're suggesting won't solve your problem, but so far none of the objections you've listed are insurmountable obstacles. They just take some configuration to make them the way you want.
I also think it's unlikely what you're asking for will be implemented, because it's a very particular request with a lot of nuance to it. If there is a solution, it will probably have to utilize the current features and may not work exactly the way you had in mind.
Anyway, if you don't want help figuring out how to make the current systems work for you, I'll leave you be.