Wow, that's a big list for starters! First off, welcome to the forum. I won't try and answer comprehensively and I'm sure others will chip in and expand the discussion.
Everybody has their own preference. The only way to find out is to try them both.
Is it necessary to prime a kit for Acrylic and Enamel paint?
I wouldn't regard it as a necessity, although I'll never be convinced that aqueous acrylic paints have enough sticking power to adhere without a primer coat to give them a key. Priming is useful in giving you a uniform layer as a starting point, this is especially the case where you might have areas of filler, or multi media construction. A primer coat will also show up any joints that might not have been filled and sanded 100%
Do you bother with washing the kit before painting.
It's a good idea, especially if there's obvious contamination with release oil. I'm usually OK with a good wipe with a cloth dampened with isopropyl alcohol.
Do Humbrol Acrylics deserve the lesser reputation?
In my opinion, no. Whilst I rarely use them for the main part of the paint job, I'm happy to use them for detail painting.
As with paint media, the only way to find out is to try different glue/cement and find which ones work best for you.
What is best primer for Acrylic / Enamel ?
Again, this is subjective, it's a matter of trying out different media. You can't go far wrong with auto paint aerosols.
If the decals are applied to a good glossy surface, no, but it doesn't hurt to use it. If you check out the Humbrol video they claim you can use Decalfix to place decals on a matt surface. You can, but personally I prefer to place decals over a glossy surface. I would recommend diluting Decalfix, especially if it has a strong whiff of ammonia.
Which combinations of finish must be avoided - i.e. finishing varnish
Generally speaking it is a good idea to avoid mixing media, but with experience you will find out which topcoats can be used with different types of paint media.
I hope this helps.