pushing a quilt….

Is it really going to be 5-15 degrees at night?

Is it a requirement that your bag be rated 5-15 degrees even if it won't be that cold?

Will they not let you come if you don't have a bag so rated?

If it is going to be that cold, maybe it's not a bad idea to borrow one of their heavier, warmer bags. I've found that once you get your pack weight down, you are freer to add a pound or two here and there for various reasons without pain.

Will, I think it depends on the insulated clothing.

I doubt you're going to get there with baselayers and fleece.

What kind of insulated clothing options do you have to work with?

I haven't done a NOLS trip but I would be thinking about condensation more than anything. Assuming you are on a long trip in other people's shelters you may not know how your bag will perform. I got the foot of my down bag wet in my friend's tent because I got pushed up against the wall at night. Something like that might happen to you over a long trip in crowded tents. If your bag is already barely enough you don't want to lose loft (of course last time I checked NOLS required 10 degree bags for summer trips so you might be fine). If you can sew why not make a quilt with 7.5 oz insulation? That should be safe and lighter?

no sewwing abilty/means, so I'm stuck with what I've got. I think I'm just going to go with renting one. Its a month long trip in the Yukon, so I'm guessing water and condensation or going to be huge issues with down. And it might get that cold, were going to be doing some mountaineering.

If they thought it was inadequate they would force me to rent one, maybe I'll bring it and see.

I think that's a good idea. Just bring it and if they allow it then great. Never hurts to ask.