The mechanism needs not go via the most abundant species. On the contrary, many times mechanisms go via less abundant species, as they are more reactive. The nature of the actual acid protonating the double bond is quite irrelevant. I would recommend you to forget about Markonicov and concentrate on carbocation stability. The stability of carbocations follows the general order tertiary more stable than secondary more stable than primary.
Conjugation also stabilizes a carbocation.
Benzylic and allylic carbocations are more stable. Yes , in several ways. 1- The stability of a carbocation is greatly decreased when it is not able to be completely planar, for instance on a bridgehead position. A 1,2-alkyl shift is under thermodinamic control and is greatly facilitated when it releases ring strain. The above considerations equally apply to reactions in the lab and to the biogenesis of natural compounds.