Clear coat over Model Air acrylics,

Gloss, Decal, Gloss again to seal, then weather & finish coat.

You will be fine with Future/Kleer thin coats only.

For a semi-gloss, allow to dry & re-coat for a glossier finish.

You can also mat Future down with up to 5-10% Tamiya X21 Flat base.

Spray thin coats of neat Future, then a coat of Future + X21, for a mat finish.

NB, I have used this with OLD formula Future/Kleer, not the new milky or smelly fragranced stuff.

I've also used Humbrol Enamel, Mat, Gloss & Semi-Gloss, which are fine over Vallejo ModelAir, provided it is fully cured. Some people report that the Humbrol Varnishes will still yellow white markings over time, but I haven't noted this yet.

I Haven't used MM acrylic varnish so can't comment.

I airbrush future as is, no thinning requred.  Clean with water and/or acrylic paint brush cleaner available in any art store.  Just make sure you really, really clean it well otherwise it you will spend some time  trying to desolving dried future with laquer thinner or such the next time around.

As for matt or satin, Vallejo air, microscale have all worked well for me.