I didn't address your other questions.
In Ex5:20, paga means to meet, or to clash into someone. There is no need
for preposition be, the correct preposition is, indeed, et, with.
In Ex23:4, paga is employed in partitive genitive, "ox of your enemy." As
you know, preposition shel is an innovation, and ancient language
generally used no preposition in this case. So no need for be, either.
Your correctly note that in Is53:12 there is no phrase relating to whom
the purported intercession is addressed. Hebrew does not routinely employ
structures without antecedents. Therefore, probably the meaning is not to
intercess, but another word, which does not require a second preposition
besides le. And this meaning is "to be attached."