Ok for garage door chain to sag on one side when door is closed

It sounds like the chain might not be on the sprocket that drives it. Get up on a ladder and look at the top of the unit. You should be able to easily see if the chain is off the sprocket. Try running the opener while you are up there too.

If the chain has come off you will need to loosen the chain tension, place the chain back over the sprocket and tighten it to the correct tension. Your user manual should have details on how to perform this but usually there is a screw you adjust on the trolley that controls the tension. Make sure to disconnect your door from the trolley before you do this.

Here's YouTube video that shows the process.

Many times the most cost effective and time effective (and "time is money" cost effective) solution to strange problems with an old opener is a new opener.

You can, of course, replace parts; but you can quickly exceed the cost of a new opener, and parts often come with no guarantee, while a new opener generally has one. Since you KNOW your opener has been abused, that's another tick in the "reasons to just replace it" column. Recall that it is a device that can kill people if it malfunctions in certain ways.

I will also suggest (from experience) that you provide a surge suppressor on the outlet serving the opener.