Are there optical or light issues which have an affect on group size? Do environmental issues such as humidity effect the consistency of sight picture at long range?

Not sure what distance you consider long range, but the official Long Range Matches as per NRA, begin at 600 yards and end at 1000 yards. Certainly not distances for hunting purposes, perhaps varmints, etc., but the light conditions are definitely a factor especially in using micrometer/peep sights. It will vary from dull overcast, best condition for my eyes, to bright sun. Problem is that over these ranges it will fade in and out with cloud cover, dull to bright while shooting and it will alter your performance if not careful to re focus. Under those conditions those adjustable rear apertures are used to maintain contrast between bullseye and your front sight picture.
Scope shooters do not have as much of a problem, but it is still there. As noted, mirage is both a negative and a positive in that you can use the direction of the mirage to somewhat detect the wind conditions, but if it is "boiling" upward you should wait and look for it to stop. If you must shoot through a "boil" be prepared to have the round go a bit on the high side, but if you do shoot would suggest you come down a click or two to keep it in the 10 ring. While checking the mirage that has been there while shooting and suddenly it goes away is a sign that the wind has picked up and some say after 10mph, it goes flat line and not seen. Check the flags, tree limbs, grass, weeds, "puff smoke," etc.
I am not capapble of giving reasons why the humidity is a factor, but good source on the matter is in Sierra's reloading manual and sure others have the same dope. It is a factor though for you will note test data specifys humidity, altitude, temperature, etc. All of those things alter performance of the round.
Those hot sticky days also work on the "trigger puller" to a great degree as well.