Re: Is a tensioner necessary?
You'll find two schools of thought on this. Some will tell you that the tensioner isn't necessary and to get rid of it. Others, me included will offer that not having a tensioner will leave you little room for chain adjustment. The original chain, #415, supplied with most kits is known for premature wear and therefore loosening. With no tensioner to take up the slack you're left with what you can get from your wheel mounting area, (drop-out) and sometimes that isn't enough so then you're faced with shortening the chain and/or adding half links to get and keep the tension right.
If you decide on using the tensioner there is an easy fix for the tensioner bracket loosening or rotating on the chainstay. Drill a small hole completely through the bracket and bike frame and install a screw and nut, I use a 10-32 Allen head cap screw and locking nut. Others simply use a self tapping screw. Either way you must be sure your chain/sprocket alignment is correct before you secure the bracket to the frame and before drilling. A good many of us have abandoned the kit supplied chain for a good quality industrial chain, #41, available at Grainger, Tractor Supply and I've heard even Ace Hradware carries it. Let us know what you decide and good luck with your decision.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"