* The darkest color I have in hand is aronia. (where are you located ?) I like blackberry and black raspberry as coloring agents too but would rank them as about 1/5 the intensity.
* 13% alcohol would not
Kill 1118. Most yeast would tolerate up to 15% so it is a risk, but ought to work.
* I see blueberry Merlot in the wine club. I like the mix so am tempted to say Malbec would also be OK.
* have you heard the phrase “oxygen is the enemy of alcohol” ? ? As a guess if you aireated your wine enough to grow yeast you should be able to taste acetaldehyde. If you are still tasting lots of tannin you might pull a refermentation off. On my part I would be tempted to do a separate 5 liter fermentation and not risk the Malbec. Option two would be go to my freezer and see how much aronia is there. A quart of juice could be sterilized then added to the Malbec. 2023, over a year old, ought to have the yeast settled out so accidental refermentation ought to be a low risk. (there also is black raspberry juice)
* I have worked on the lab bench so I would consider one to five ml of almond extract to up the flavor. I also would try a chocolate extract. Without adding vanilla the chocolate polyphenols would dominate, give a slight impression of chocolate but not be overpowering. (Making your own extract with cocoa powder is messy but easy.)
* I like tannin in wine. Are you satisfied with the astringent notes? As an experiment I did a mulberry juice with the maximum package direction of grape tannin. It passes as a “big red wine”. ? Scott Labs finishing tannin is good, but they have too many choices.
