Sounds like you are having the usual nightmare! - what seems like a simple job is usually the one that causes the most trouble!

Yes, the sprocket nut turns counterclockwise to undo, but typically they are torqued up VERY tight - using a long handle socket bar (ie 600mm or more) can help, plus holding the whole bike on the rear brake as edteamslr and I suggested - do you have someone who can sit on the bike and hold the rear brake down while you do it?

The other option is to use a pneumatic impact driver like they have in a commercial workshop/garage - that ought to free it off...

As for the chain - a lot of o-ring chains only come with a rivet link (rather than the clip link you refer to) these days - and a lot of mechanics will recommend you only use a rivet link on a larger capacity machine - especially a single that has a lot of torque.

I've found you can use a split link with no problems (you can buy/order them from any bike shop - just make sure you say which chain it is for, as some have different diameter pins - I believe DID are all the same though) - although a good trick is once you have fitted the link, cover the clip and outer plate in superglue or epoxy - then that won't be coming apart any time soon!

Good luck!

J xx

ps. long term it might be worth investing in a chain tool that can remove pins and press/rivet links too - you never know when you (or someone else) might need it. I recommend the Motion Pro tool - not cheap, but far better than the cheap ones you find on the internet etc.