1) Using a sling may indeed increase barrel wobble, but you can combat this in a few different ways. Mainly, keep all the screws tight and clean. A metal body and/or reinforced 1 piece barrel can't hurt either. I think any increased strain on a weapon due to a sling is worth it because of the things you avoid by using the sling; ie Having to set down your weapon, dropping it, etc.
2) Wargameclub uses frames. If your going to link to something on there, open it in a new browser window to see the correct URL.
3) There are laser pointers that will work during the day, but you're looking at a lot more $. I don't feel laser pointers are useful enough in airsoft to necessitate carrying around. Day or night.
Just my .02