Your chain has cancer, if you're going to ride a lot a new chain will keep you from the walk of shame. You cleaned the surface rust. Where it matters, you'll never get rust free, it's deep.
The chain is two weeks old lol
I would soak it in some WD-40 to get rid of that rust and wipe it down and then really soak it good in some 2 cycle oil and then wipe all the excess oil off of it.
As far as the chain length, see if it MAY be possible to just get rid of the tensioner altogether like what I have done on my bike...You might have to adjust things like bringing the rear rear a little more forward or backwards etc to do this.
Here is mine with no tensioner after I did it for mine with two different motors...I have that same 36 tooth sprocket in the rear like you do...I also had to custom maket the pedal chain to size when I did this to make it match the drive chain...I also use the 22 tooth freewheel sprocket on the pedal side as I got rid of the coaster brake wheel...It is also one less thing to go wrong as well...lol...Just another additional thought if ya like it...lol.
