Questions I have are :
1. I have a stock Solo no modes not a greenbox or whatever that new thing is called.
That's not a question, that's a statement. I take it you mean you don't have a GreenCube?
2. Are opensolo (latest release) and Solex mated and meant to be used together? all set to go? or is the stock firmware in some way still used or expected to be used?
You're just cheating here by asking multiple questions in one line.. I mean come on... No Open Solo and Solex aren't "mated". You can install Open Solo and still use the 3DR app (although why would you want to? Solex is so much better..). The only reason a lot of people mention Solex and Open Solo at the same time is because updating your Solo to Open Solo is very very easy to do using Solex.
3. My understanding (yes I have read some things) is that Solex will update a stock solo to the lates OpenSolo either required or optional. (so maybe scratch question #2).
It's optional, so you can still fly stock if you want or upgrade if you want.
4. Do all the buttons/commands work just like the stock software on the TX/Controller or are there differences that I'll need to get familiar with or setup myself?
Do you mean "Do any of the buttons/commands change once upgraded to Open Solo?" Yes and no, they are all mostly the same, they just support extra functions via long hold/short hold. The release notes HERE will explain more about that.
5. Is my old Galaxy S5 with Android 6.0 plenty to run it happily?
I used to run it all the time with my Galaxy S5 with no issues, however it will depend on the state of your device (how much bloatware is on it, how many background apps are running, are you cleaning it of trash files on a regular basis). If your phone is in a nice healthy state, you shouldn't have any issues.
6. Do you pretty much NEED Solex to use Opensolo or does it work just fine on the stock app and other free apps?
No you don't NEED Solex to use Opensolo. However Solex offers more functionality and reliability compared to the stock app. There are features on the stock app that simply don't work that work perfectly on the Solex app. There's also additional modes available on the Solex app that the stock app just doesn't offer.
7. I'm considering buying it anyway to support opensolo/Solex even if it does not really do a whole lot, but and extrememly curious what the benefits are compared to staying on any of the stock software.
I'm not sure who told you that Solex doesn't do a whole lot, but maybe you should watch some of the youtube video's regarding the capabilities of Solex. Optical Follow, Boat Mode (if you have a Green Cube), Return to me/launch (return to me never worked on the Solo stock app), control over the lighting, Find my vehicle, the ability to record export and share cable cams, multiple vehicle and battery logging, Optical follow, inception style smart shots, tap to fly with optional purchases you can also do mission planning and geotag photo's. The list goes on and on...