I'm not an expert, but I've installed 3M Scotchcal (the one from x-pel) in various parts of my car. So, this is based on my observation on my car only.
(1) Air bubbles:
(1a) If the bubbles trap the alcohol solution, they will go away. This seems to be unavoidable when you install the thicker sheets such as those for the headlights & fogs. Mine streaks pretty bad in the first 2 weeks, but it's perfectly fine afterwards. The time it takes for these streaks to dissapear depends on the sunlight exposure. Based on this, apparently the alcohol solution does evaporate through the 3M sheet.
(1b) If the bubbles trap air, then it will not go away. If I recall correctly, there is a "bubble remover" tool (I believe x-pel sells these). It basically punches a hole so that the air can be removed.
(2) Stretch marks:
I'm not sure to what extent these marks are, but it seems like it's quite difficult not to have some stretch marks appear when you're working on highly curved surfaces (e.g. The leading edge of the hood scoop, going into the scoop inlet). As for how much is considered "sloppy job", and how little is considered "as good as it gets", I can't really tell. I know I could prevent stretch &/ fold marks in parts like hood, side doors, top side of the bumper, etc., but the hood scoop inlet seems to be one of the tricky parts. If you're talking about a stretch &/ fold mark in a very large surface such as a hood, i.m.h.o., it could have been prevented.
The good news is that since these things are easy to remove, you can always redo a part that you don't really like.
my 2 cents