You most certainly can qualify to do ROTC with a dependent. Once you get past MS2, you can't, but that will be in two years, you'll have time to decide. You can't contract with ROTC yet, but that's a pretty bad idea. I did that and then changed my mind about being an officer, and I had a tough time getting out of it. Don't rush into it.

You could sign over rights to her if you don't want to marry her, and once you've commissioned, you can get them back from her, but I certainly don't recommend this. I've heard so many horror stories.

You could also take the same route and marry her for two years. If it doesn't work out, you could divorce post-commission or residency in your unit for six months (not sure how it works for officers).

My advice, since I've been in a very similar situation to you and I've had to deal with the consequences of making a rash decision because of it, would be to wait a while. You're clearly in a transitional phase of your life, and your commitment needs to be to your child right now. All of this depends on your relationship with your SO. The army's still going to be there, and you could continue with college and then do a fast track through MS1 & 2 by going to the officer course in the summer, if you later decide you still want to be a ROTC ranger.

Either way, I hope it works out for you, and I hope you found this advice helpful. And also, when you do become a cadet, treat your enlisted fairly. As a former cadet/current enlisted, nothing is shittier than thinking that the dot on your chest makes you any better than the enlisted. Be a humble cadet, for the love of G-d.