I've no idea about how to answer your question. I'd also like to know the answer if possible, but I don't have the time this month to do modding.
However, here's what I know/would try:
a. What determines wether a symbol is in the first or second row ? and How can I mod that ?
Try comparing a vehicle to an infantry unit and see if there's something different in their squad_ui.
Alternatively, try changing a vehicle type from vehicle to infantry (just copy the type_ext from an infantry unit to a panzer for instance).
It could be, for instance, that the three rows are hardcoded so that the 1st one is for infantry, second one is for vehicles, etc.
Apart from brit emplacements, howitzers also belong to the 3rd row. Thus, also check what's different about them. The OKW doctrinal flak cannon might also belong there.
b. Do the symbols that appear in the upper side of the screen work only with sbps ? or could it be moddable for ebps also ?
I don't know. However, my best guess is that since those are squad shields, they are tied to sbps.
c. I built a bunker entity and now I want to give it a symbol. Then we created an sbps for it and tried in many ways to add a symbol but we failed. The idea is this symbol should appear in the upper side of the screen to be selected in a easy way during the game.
I've also failed that many times. I've tried it with the OKW FlakHQ for instance (e.g., by copying what Bofors does), but nothing worked.
What I tried (and failed) to do was copy Bofors sbps and assign flakHQ under the unit list and then prayed that this would work.
However, in retrospect I didn't try playing with the FlakHQ ebps much to adapt it (e.g,. type_ext). Thus, the key to solving this might be there (if this is even solvable).