Regarding "precursor" vs "pioneering": No, they are not interchangeable with regarding to meaning. A precursor is a relationship between two things. In your example, the "instrument" that came before the guitar is the precursor. However, if the guitar had never been invented, the "instrument" would not be a precursor. So, another definition could be: "This which led to that." No that = no this leading to it. Also, precursing isn't a word (until someone says it, I guess!) and it is most definitely not a verb. One does not precurse, unless you are about to swear, maybe. As well, precursor, the vast majority of time, does not refer to a specific person... I'm guessing because, outside of reproduction, this person does not lead to that person. Again, that's not a hard and fast rule, but one doesn't say, "Captain Cook was the precursor to the discovery of new lands," but instead, "Captain Cook's voyages were the precursor to the discovery of new lands."
Pioneer(ing) is a noun to indicate (usually) a person. It's not limited to that at all as a kangaroo can be a pioneer depending on the context. However, "It is a pioneer," does not ring as proper as, "She is a pioneer." Again, not to say it's not correct, but a pioneer is one who pioneers. It's a noun defined by a verb. One cannot be a pioneer unless they are pioneering. It's like saying one is a liar if one never lies. You have to lie to be a liar. You have to pioneer to be a pioneer.
That said, unless something can pioneer, it cannot be a pioneer. Perhaps a dinosaur pioneered onto the glacier, which would be correct, but a glacier cannot pioneer. It's personification.
Regarding the word, Pioneering, specifically, it does indicate relationship, but that of chronology, not of succession. Precursor is, "this, which led to that." Pioneering is, "they who did it first." The relationship is between "they/he/she" and whatever "it" is. So, "He set foot on the moon." But it must be the "first" or some similar relationship. So it's not a successive, "this came before that," but, "this was the first ever."
The precursor to the spaceship was the airplane whose precursor was the boat whose precursor was the wagon whose precursor was the foot. (I'm making that up, but you can see where I'm going with that, I hope).
However, the pioneer of the spaceship was not the airplane, nor was the airplane pioneering the spaceship. Astronaut X was the pioneer who flew the spaceship for the first time. Lewis and Clark pioneered the West.
Sorry, that was wordy but I hope it explains that these are not interchangeable words.
Regarding, "a precursor of," and, "a precursor to." They are interchangeable. As I mentioned "precursor" indicates relationship. Both of and to properly refer to "that" which followed.