Primer. There are many opinions on this! If your plastic is clean and grease free (that includes fingerprints!) you shouldn't have to prime. Now,having said that, I do prime prior to applying acrylic paints. There are two reasons. Firstly the paint does adhere better to the primer than to the bare plastic in my experience. Secondly a primer coat will show any flaws or blemishes in your model before you spray your camo scheme and allow you to fix them much more easily at this stage than later.
Achieving invisible seams and a food paint finish are precisely the basics I was referring to earlier. These things are much more easily said than done,even the most experienced modellers come unstuck from time to time,I certainly do.
On your second question you should apply your top coat and allow it to dry AND cure. Your acrylics will seem dry in a matter of minutes but you should leave them at least overnight before putting anything over them.
Next apply a clear gloss coat. This will provide a smooth surface for the
decals and minimise the chance of "silvering". Silvering is when air trapped under the decal or even (microscopically) in the surface of an uneven paint layer prevents the decal film from becoming invisible.
Once that clear coat is dry you can apply your
decals. Do whatever you need to do with setting solutions etc before proceeding to the next step.
When you are happy with your
decals and everything is dry apply another clear coat. This will seal your
decals in and protect both them and your paint job from the weathering.
Now,finally,you can move on to weathering.
Take your time,there is no way to rush this. If you don't allow things time to dry I guarantee you'll make a mess! It's not as difficult as it seems when it's written out like this,I'm sure you'll be fine. Any questions just fire away,and good luck with the models.
Cheers
Steve