Will try to answer everything, do not hesitate to respond if I missed something.
If I have a wine that is bulk aging, is clear, stable and I decide I want to add a chocolate kick I commonly add 1 cup of Hershey's Syrup or the Special Dark Syrup per gallon. I have not used any other brand of syrup. Just remember if wine has not been stabilized with k-meta + sorbate it will likely referment due to addition of sweet syrup. The wine drops clear again quickly, less than two weeks, and I then rack. You can enhance the chocolate by adding 2-5 drops quality vanilla extract per 750ml bottle.
If I know I want to incorporate chocolate from the beginning then I add it in primary. Usually 4 ounces by weight per gallon, a Dutch processed cocoa powder is preferred. Its acid base is more balanced and will actually need less time to age compared to a basic cocoa powder, like Hershey's natural unsweetened. Do all your acidity adjustment on the backend because the cocoa powder messes with everything up front, or adjust before you add the cocoa powder. You will get immense chocolate foam in primary, allow plenty of headspace. Plus, you will develop gross fudgelike lees. You will stir the cap like normal and rack off gross lees like normal THOUGH you want to keep fruit contained as much as possible because you want to let the finer chocolate lees remain so the components break down rather than removing them all prematurely. I find that I may rack 3-4 more additional times with a cocoa powder based wine versus one without. Two rackings to deal with gross lees and one extra racking for fine lees. I find it takes around 8-9 months after reaching FG for a whole fruit based wine, which has cocoa powder added, to clear; even less for a 100% juice based chocolate wine. But, since they need to bulk age it really matters not, in my opinion. I just make sure my fruit based lees are dealt with and then sit back and relax.
With nibs, I put them in secondary for up to 12 weeks. Would do the same for solid chocolate (if I opted to work with it). The use of milk chocolate, due to its casein content, will act like a fining agent, so you usually see sooner clearing. I also add instant espresso granules or freshly roasted coffee beans to intensify chocolate component--amount varies, when to add primary vs secondary... still experimenting.
Chocolate mead is delightful, and every batch I have made clears so much earlier than wine. Talking in less than 90 days.
I have added a chocolate extract, Star Kay White is the preferred brand, found online at Olive Nation. And I will add it drop by drop to a glass when I want to see how it would taste. No reason why you cannot use extract unless you plan to enter competition (some may not allow).
As far as aging, handle like any other wine. If you bottle prematurely and wine is not clear, do not worry. I find some of my earlier-in-the-process bottles have a fine chocolate sediment and I just swirl the bottle or decant, just depends. And my favorite is made using Droste cocoa powder. Chocolate raspberry, chocolate raspberry chambourcin, and chocolate chai mead are among my favorites, plus it is so easy to go port-style with these wines.
