Really long story, but I figured I'd share for anyone interested in how buying certain carrier models works on other carriers, or anyone interested in free cellphone service.
I've been checking Apple's website almost daily for stock on the 64GB silver iPhone SE. I looked for every model except Sprint, because the Verizon, AT&T, T-Mobile, and SIM-free models are all the same.
Ironically, I plan on using it on Sprint the majority of the time since I get free service with them, but the non-Sprint model works fine on Sprint; it just doesn't have band 41 so it can't use Sprint's LTE Plus (Spark) network for the fastest speeds possible. Sprint doesn't have LTE Plus in my area anyway, and the Sprint model is missing essential Verizon and AT&T bands. I have Verizon as my main line, so I would like the option to put my Verizon SIM in occasionally.
Anyway, yesterday I checked Apple's website and it showed "ships to store" for SIM-free and "unavailable for pickup" for all other models except AT&T at the second nearest Apple Store. It was just over an hour away, but it would be worth the drive if I could get it. I called the Apple Store to confirm they had it and they did. I explained I'm on Verizon but wanted to buy the AT&T model, and I wanted to buy it on the payment plan where I trade in my iPhone 5s and get the SE for $14.16/month. The person I spoke to said that wouldn't be a problem. Then I asked if they could hold one for me since it would take an hour to get there but I was heading out right then. He said he couldn't make that call but transferred me to someone who could. They told me no. ¬_¬ But she said they had plenty in stock and there should definitely still be some there when I got there.
So I headed out. An hour later I get to the store and someone immediately comes over to help me. I explained again how I'm on Verizon and wanted to buy the AT&T iPhone SE on a payment plan but I'm on Verizon. SHE TOLD ME I COULDN'T DO THAT!

She said when you buy it on the payment plan, they have to activate it in store, and being an AT&T model, it has to activate on AT&T. I told her I called ahead to confirm I could, so she told me she'd get someone else to explain it to me because she must not be doing a good job. So the guy that comes over to explain it to me says Verizon is CDMA and AT&T is GSM. The phone is just not compatible because the AT&T model doesn't have the required hardware. I told him I can use my dad's AT&T iPhone 6s on Verizon, and he said that's because it's a different iPhone. The 6s can do it but the SE can't.
I pulled out my phone to check Apple's website. I was 99.99% sure all carriers were the same model. Only a tiny fraction of doubt now that an Apple employee is telling me otherwise. The first girl asked me if I wanted a minute to decide what I wanted to do and I said yes. After pulling up apple's LTE page, I see the Verizon, AT&T, and T-Mobile iPhone SE's are all the same model. So I walked near him to talk to him again. He was busy with another customer, so another employee asked if I needed anything. I said I just wanted to talk to him. She said there were plenty of other employees who could help, and one walked over so I just decided to explain the situation to her.
Not trying to be ageist here, but the first girl and guy who both told me it couldn't be done looked like they were around 20. The lady who was helping me at this point looked like she was in her 60's. After explaining what just happened, she said "I don't know why he would have told you that. It's my understanding that the AT&T model would work fine on Verizon." So I've got some 20 year olds, who should be good with technology, telling me it can't be done, and a 60 year old woman telling me it can. And she was right.
It took about 30 minutes to get everything taken care of, but I walked out of the store with my brand new 64GB silver AT&T iPhone SE.
SHE DIDN'T EVEN OPEN THE BOX! I wish I could give this woman all kinds of praises. I wish I got her name because I would definitely call Apple and let them know what a great employee she is.
So I got out to my car, opened the box, activated the new phone on the AT&T SIM that came with it, then switched my Verizon SIM over and everything worked great. I got home and restored to from an iCloud backup. Then, I checked RingPlus' website around 9pm and found they just started a promotion for the next two hours for 3000 minutes, 2750 texts, and 2750MB of data for free every month. They use Sprint, so not the best, but free service is free service. I had to cancel my old line which was tied to the iPhone 5s I traded in, so I lost the $10 I loaded on the account, but it was worth it for 2.69GB of free data every month.
So now I have a Galaxy S7 Edge with Verizon and unlimited data, and a backup iPhone SE with 2.69GB of free data every month. There's a $25 top-up fee, so I did have to pay that to get the free data, but as long as I don't go over 2.69GB in any given month, that $25 is never touched and I get 2.69GB free every month. (Plus 3000 minutes and 2750 texts, but I'll never use that.)