With a tensioner the chain is sitting higher, that is why the chain will not rub. On my first bike I did not use the tensioner because I was able to put washers on the rear wheel shaft which spread open my bike frame enough to clear the frame for the chain.
You could also see if flipping the rear sprocket so that the chain runs closer to the rear wheel may help, or even leaving the rubber off of the outside of the sprocket may help.
From what I have seen, every bike and rear wheel is different and it could be just a matter of spending days and days screwing around until things work out for you.
If you look around you may notice that a lot of belt driven machines use what may be called an idler and they have bearings. It could be as simple as replacing the ones that come with the bike kits with one of these off of a snow blower of garden tractor.