What problem is you trying to fix?
If shopping and support is difficult for you, follow the old saying "if it aint broke, don't fix it".
A 6-speed can be both freewheel and freehub.
6-speed freehubs won't take 8-speed cassettes.
Some 6-speed freehubs can be replaced with 8-speed freehubs.
Will take some homework.
There are 8-speed freewheels, but they are held to be prone to axles bending.
Probably easier to find another wheel.
Using only one out of 2/3 chainrings is a so-so thing.
(Externally) bikes are designed to let the chain move with relative ease from one gear to the next. Now you want the chain to stay put. It's common to have to add either chainguides or to use special chainrings to keep the chain in place.