You are in process so some of this isn’t useful.
* fruit solids give flavor, if my goal is to make knock your Sox off flavor I will add any available bland fruit concentrate, could be passion fruit, could be apple, could be white grape. Sugar with added nutrients doesn’t give much fruity impact, the up side on sugar water is that it lets you lower the total acidity and balance sweet flavors.
* the break point for yeast is around 2.8 pH, you could add yeast now with the risk that it gets stuck.
* I don’t look at getting stuck as bad, if you are fermented down to 1.005 it has residual sweetness which helps bring out fruity flavors. The key is to not deferment making a bottle bomb. Sitting with no change in gravity three months at 20C should be stable. ,,, and Oh yes pH 2.9 will be more stable than 3.5. and ten percent ABV is quite stable.
* always keep the head space low, oxygen is the enemy of ethyl alcohol making acetaldehyde, ,,, burn in the back of the throat swallowing.
* baking soda is sodium bicarbonate. Technically you can push the pH up with it. The down side is that it is sodium which will give a salty taste. Most folks don’t like salt in their wine.
* calcium carbonate (basically ground up limestone) has a clean flavor. If you are really anxious this is limestone in a plastic bag. ,,,, ie the stuff used in making roads ,,, old technology with concrete tanks would be dissolving the concrete raising the pH.
* if tannin is in Vinters Best you will taste either bitter or more likely astringent flavors. As a manufacturer it is extremely unlikely that I would add tannin since you/ the buyer will taste it. Dark color can mean high temperature in the evaporator or it can mean sitting in a warehouse for two years.
* I really like polyphenols and add to many wines. ex this week I started a strawberry with 800 grams of crab apple juice in it. Tannins are the magic which allows red wine to survive oxygen/ sloppy air exposure. The crab I use gives astringent notes without bitter. There are a lot of tannins available that don’t give bitter. ,,, taste before you add ,,, low level you can add bitter if that is all you can find (ex 1/4 package direction) If ordering look at BlancSoft or read the Scott Handbook for flavor notes.
* two days? refrigerated no problem, yes folks do room temp but it has risk. When yeast is growing it needs to be above 10C.
* passion fruit is neat aromatics. That said bad yeast nutrition can push H2S and other reductive sulfur which are powerful (ppt levels) and will mask the wonderful aromatics. I am a fan of organic nitrogen (ex Fermaid) since it works better/ at lower measured YAN than DAP.