Been snowed on three of the last five years last week of September at elev 10400, near Telluride. 1" to 4" on the ground, one morning was 19 degrees . Plan for the opportunity of snow if you will be above 9k.
No one can say, depends on you and the weather and your ground pad.
During 2nd rifle I use a -20 bag, that’s too much for some. I can always open it up if I get hot, but I got tired of freezing at night. Yeah I run cold.
If you are worried, make sure (as has been said many times) you have a pad between you and the ground or cot, consider a light fleece blanket, have a wool beanie for your head, and consider a cheap bivy sack. The bivy sack can add 10 degrees of warmth to the bag although you may get some condensation in the bivy overnight requiring you to pull the sleeping bag out to dry in the morning.
I’ve personally froze my ass off more than a few times. When tags mattered more than gear. I was running a BA encampment 15 and a Klymit pad. It got down below 10 degrees in Frank Church in 2017. Not a fun experience. I literally had on everything I had with me and woke up shivering. Never again. I got a Nemo Sonic 0 degree and a therma NeoAir xtherm. Best thing I ever did!
As a lot of these guys have said you can make it work if you want to sleep in your base layers and a beanie. But if you have the money I would recommend getting a bag rated to 0 degrees, that way you dont have to faff around with a sleeping bag liner and sleeping in a bunch of extra clothes. I have a marmot never summer rated to 0 and I love it. There are better brands and warmer bags but for around 300 it's a pretty solid investment. I spent many years using lighter bags with lower temperature ratings and never slept as well. For an extra pound or so of weight a 0 degree bag or lower will be something you want to have at that time of year.
A tag. It was sept 17-21 it was cold AF! Which it is typically not that cold but 2017 got really cold. It only takes 1 trip like that to air on the side of being warm and personally packing an extra 8-16 oz doesn’t bother me.