I have recently read about a method replacing Future, using Tamiya Clear thinned with Mr. Leveling thinner, then immediately misted with Mr. Level thinner straight. The question is, can you then use enamel based washes directly over it?

In a word, Absolutely.Mr. Leveling thinner is essentially a lacquer thinner with a retarder in it. What it does to the Clear is sort of "melt" the fresh paint and letting it level out smoothly. You can also do the same technique with Tamiya XF paints if you originally laid them down with Mr. Leveling thinner. I have done this before and it completely removed the need to gloss coat my model.

One thing to keep in mind is that because MLT dries slowly, the paint layer will be slightly tacky for a little bit longer than normal, so watch where you put your fingers!

Yes, but once dried, can you apply enamel based washes directly on?

I've never tried this technique in particular but it never happened to me and I never even heard of problems between Enamel on Laquer, the opposite can give problems if too wet

personally I have never had any problem even with enamel on acrylic or enamel on enamel and often i avoid any clear coat if i can, I do not find them strictly necessary if not where I put the decals

Another option to consider is using lacquer based paint to paint your model. After letting the paint dry thoroughly there is no real need for a clear cote. It's lacquer, same as most clear cotes, with a very durable surface. I've been using enamel washes on Tamiya lacquers for a few years, and more recently with Hataka, with no problems. The trick is to let the paint dry for several days. Even though the lacquer surface is dry to the touch in a few minutes and can be handled in less than an hour, the underlying paint takes longer to dry. I understand the reason Model T's were all painted black was because they used lacquer based paints and black dried quicker than any other color. At that, I think it was something like 30 days before the paint was fully cured.

Yes, not an issue (with a caveat, below). I use Gunze and Tamiya clears (depending on what I have on hand) thinned with Mr Leveling Thinner and it works great. I then weather with oils/enamels.

The caveat is you can't flood the model with thinner. Enamel thinner (white spirit, turps, etc) will eat lacquers if you use enough/too much. But if you are wiping away panel liner, paint, etc it's fine. I use a brush or cotton bud, dipped in thinner then dabbed on a paper towel to remove most of the thinner. Never had an issue. I also use Tamiya panel liner (because I'm too lazy to make a wash) which hasn't caused issues.

Beginning to like Tamiya clear coats, as they come in all three kinds, but them damn dreaded white spots that show up from the Talc used in flat finishes is the lady.
Dull coat included.