Re: **important Questions**
The stock chain is scrap don't use it, there are many quality chains in the right size. The "rubber donut" sprocket mount is a practical joke. It can and does work but it is such a poor design. The chain tensioner can be omitted if you can get your pedal and drive chains to tension at the same axle adjustment.
Good chain doesn't stretch much (I've never removed links from any chain after it's been mounted).
Rigid/mechanically mounted (not sandwiched) sprockets are the way to go; they spread the load over all the spokes instead of one side.
Chain tensioners as ghosto said need to be in line w/ the chain, sprung or not. And really aren't necessary w/ quality chain and maybe 1/2 links!
There are many solutions!