Chainrings, Cassette & derailleur MTB

[Ajax Bay]

Normal rule of thumb is that you're fine going one speed either way.
Jockey wheels (suspect yours are fine, with a clean): here's part of a post from a couple of years ago:
My ninjas needed replacing (see image; includes relatively new one for comparison). I suspect that they were the originals (ie with the 105 RD) so had about 20,000km on them.
Replaced with basic BBB sealed roller bearing ones (£10 the pair from my LBS).
On advice from my LBS I prised off the seal on the new ones (tip of Stanley knife: clicks off easily and clicks back on fine) and thumbed some more grease in there
Threadlock on the screws.

 

[Bagheera]

Normal rule of thumb is that you're fine going one speed either way.
Jockey wheels (suspect yours are fine, with a clean): here's part of a post from a couple of years ago:
My ninjas needed replacing (see image; includes relatively new one for comparison). I suspect that they were the originals (ie with the 105 RD) so had about 20,000km on them.
Replaced with basic BBB sealed roller bearing ones (£10 the pair from my LBS).
On advice from my LBS I prised off the seal on the new ones (tip of Stanley knife: clicks off easily and clicks back on fine) and thumbed some more grease in there
Threadlock on the screws.
View attachment 518401

Thanks for that. Looking at the new one on there....I think mine are OK! I'll post a pic anyway.

Tried to remove the cassette to today but hit a snag in that i didn't have the right size removal tool. That will have to wait. In trying to remove the rear axle, I had a right job getting to one nut - not sure what it is - it didn't have the usual six sides. It was Nut, spacer then this weird 'Nut'. In the end i used a pair of pliers to thread it off the axle. Here's a pic, I'd be grateful if you could tell me what it's called and if they're all awkward to get at.......photos to follow shortly...

 

[Bagheera]

Here's some pics

 

[Ajax Bay]

Jockey wheels are fine. Clean, lube and refit (ideally with a drop of threadlock).
You do not have a cassette, it's a 'freewheel' and with the right tool, the whole shabang comes off. This is likely on very, very tight.
You need to put the axle and the bearings back in and just tighten it for now.
Then fit the tool in: it undoes anti-clockwise (ie normally).
Then put the tool in a secure vice and slot the freewheel/wheel down onto it. This allows you to use the leverage of the wheel (and a good grip on the tyre with both hands) to unscrew it (from the top it's clockwise, of course).
The odd nut you've pictured is a cone nut from the hub bearing. There is normally (though not in image below) another 'lock nut' with the same flats x 2. Two cone spanners are required.

https://sheldonbrown.com/cone-adjustment.html
Review my post #10 ref freewheels and consider the 6-7-8 chainset linked, too.

 

[Ajax Bay]

Hub maintenance: Sheldon: https://sheldonbrown.com/tooltips/hubs.html