If it tastes plummy its already oxidized as a niagra should not taste plummy. The gunk in the bag was the good stuff from your must (not wine) precipitating out, they protect the wine from oxidation when they are in solution. I have used old packs of yeast, they can still be alive, activated as a starter to make sure of course, so your yeast pack my have been ok but better to be safe and get a new one.
So should you go ahead and make it? Of course. But you have to be aware that it is already oxidized some, you need to make sure you keep the SO2 levels at the right place to protect what is left, and if it all turns brown next week you will know it was already started down that path. This wont be one of those you can age a long time so plan on drinking it sooner rather than later and good luck.