What thoughtful questions!

Since fruits do vary by pH, amount of juice and texture, it is best to use a recipe meant for that type of fruit.  However, most jam recipes are pretty similar, regardless of what fruit is being used.   Most common fruits used in jams are acidic enough so that you do not having to worry about swapping one for another or even mixing them together  in a recipe, what might change is the quality of the jam, not the safety.   The known exceptions are mango, bananas, white peaches, all melons, and figs. These would all need additional acidification to be made safe for a boiling water bath process. 

You could use black currents instead of red. Some recipes are generic “berries” and the currents should work in those recipes as well.  If you compare blackberry versus blueberry recipes, there is usually more pectin to less fruit or more cooking for blackberries because of their excess juice and lower natural levels of pectin. Currents may fit the blackberry recipe better than the blueberry recipe. 

It is safe to add limited amounts of flavorings to jams without changing their safety.  Dried spices and herbs, extracts and oils are all safe in the amounts that would be used in a jam. Fresh herbs may be a bit more questionable. Some would be safe, but I don’t have good guidelines for how much it would take to change the pH and the safety of the product.   I will check further to see if there are any guidelines. 

Lemon and Lime juice can be used interchangeably.  If the recipe does not specific bottled lemon juice, then fresh is safe. In that case, the lemon juice is most likely there to enhance the flavor and work with the pectin to gel the product and is not part of the safety equation. 

I suggest that you look for recipes in packages of pectin or on the websites for the pectin manufacturers.  We know that they test their recipes and they know their products well, so have good directions to follow.  Ball® 's website Freshpreserving,com also has many choices and they are tested recipes.  They have many creative recipes to look over. 

Hope this helps!  Let me know if you have any other questions.