I'm new to mead (brewed beer up until recently). I have a melomel in secondary now. I've racked a couple times, still sediment, but the taste is amazing at 10.5% ABV. I used 3lbs honey, 1 gal water, 1 cup frozen strawberries, 1 cup frozen blueberries, and a large orange.
It's been in process for a few weeks now and I can tell it will need some transfers to clear it up and get rid of sediment. I added yeast nutrient before pitching a champagne yeast. As this is my first melomel I was mostly winging it but there's a local home brew store with a very helpful owner. I bought pectin enzyme and added before transfer to seconday, that should clear it up for me over the course of a week.
Your's is just a straight mead? If so, maybe a bunch of transfers with a few days in between to get the sediment to settle, then add whatever chemical to clarify (if needed).
If you want to back sweeten you can add a non-fermentable sweetner like xylitol, so I've heard. I don't know what to do about the vinegar taste. As other people stated, maybe some wild bacteria got in. Seems like it's easy to do with many transfers. I have a spray bottle of diluted, per manufacturer's instructions, starsan. I've sampled many times, took gravity readings, and transfered, with no contamination....so far.