If it has a shiny/reflective surface and doesn't melt or bloom (much) at room temperature or hand temperature, then it's already tempered. Virtually every packaged chocolate is already tempered.
Untempered chocolate generally needs to be refrigerated for longer-term storage, so if a package doesn't specify refrigeration (and I've never seen one that does), you can assume that it is tempered.
To answer your last question: yes.
Regarding the previous question, it's because the temperature at which the cocoa butter in the chocolate crystallizes affects the overall consistency of the chocolate. If you've ever eaten a chocolate bar that was left in a car on a hot day after it has cooled down again, and who hasn't, you'll know about this. Sometimes chocolate tastes fine but has a definite 'grittiness' to it: other things being equal, that's usually because the cocoa butter crystals are too large.
Cocoa butter has a number of different crystal forms, each of which will have a slightly different effect on texture and melting point. When melted chocolate sets, the cocoa butter crystallizes, and it will generally all crystallize with the structure that it starts to crystallize with. (This process where crystals start to grow is called nucleation.)
So you want to have control over the temperature at which crystallization starts. The optimum crystal form for nice chocolate will tend to predominate when crystallization starts somewhere between 18-25 degrees C (around 64-77F).
So if you think about it: when you add room temperature stuff (like a pretzel) to chocolate that is considerably hotter than room temperature, the resultant temperature on the surface of that pretzel will likely be way higher than 18-25 degrees. On the other hand, if you are dipping an item that is significantly below room temp, then your chances of hitting that window are much higher.
The surface structure of the thing being dipped can of course make a difference to crystallization as well, but for normal purposes that is marginal. To put it another way, it doesn't matter what ingredients you're dipping - temperature is key!
Best Answer
You cannot use sugar unless you do not mind having crunchy sugar crystals. There is no way of sweetening chocolate by adding sugar, even powdered sugar, without a conche. I think if the cacao contents are not too dissimilar, you should be fine. I have got away with 70% seeding 85%, but I cannot tell you whether you could get away with any mix. You can also sweeten your 100% by mixing sweet dark chocolate.
I would not use whole chips though, you want it grated. In fact, I would temper the Callebaut first and grate it. Keep a stash of that as seed. I have also routinely tempered dark chocolate without seeding.