1) John, I wouldn't say normal, as in it always happens, but it has certainly happened to me on numerous occasions ...
I think it has more to do with the color coat thickness than it does the clear..This is assuming your good with A/Bing your clears & not going over some parts of the surface too fast ??You might have gone over an area more times with your flat paint so its a little more pourous there.?? I am no expert thats for sure!!
You can try sanding lightly & reapply just in those duller areas & it may even out for you!
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2) Textured hmm?? Hard to comment on the paint without a pic but, going back to #1 resanding & reapplying clear to just those areas & then maybe a final clear coat over all, could enhance the look??
3) I have succesfully decalled & applied washes on a semi gloss surface, usally Mr Color semi gloss paints. If I have a good shine showing, not neccesarily super glossy, I proceed. I also, usually just run my wet finger over the surface & if it feels glass smooth all is good so I proceed with decals..pretty scientific huh?? LOL!!
4) Depends on the gloss coat brand & to some degree you A/B nozzle size.. ie.. Future I spray directly from the bottle.. Floquil & M/M enamels I use a thinner.
I have recently switched to using Tamiya or Gunze clears in a spray can. It comes out like a" fire hose" but clings to the surface like "grim death" & levels out tight...
Hope some of this helps :)