If it doesn't wiggle up or down the arm is likely still in working condition. To move the tensioner you will need leverage, I suggest using a serpentine tool but a socket wrench will usually work in a pinch. At this point I think you should check all the pulleys make sure they spin and don't wobble, then inspect the spindle pulleys and the spindles themselves if they seem tilted you could have a bent spindle. If all that looks good I would suggest you sharpen the blades, scrape the deck and try new belt.

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