1) Si jamais il venait à tenter quoi que ce soit, on serait prévenues tout de suite !
= "If he should ever try anything funny, ..."
In the 1st example, the expression "venir à faire" serves to indicate a lower possibility of his unscrupulous actions than the version without it "si jamais il tentait ..." does.
2) Si jamais il en venait à tenter quoi que ce soit, on serait prévenues tout de suite !
= "If he ever came to try anything funny, ..."
In the 2nd example, on the other hand, the expression "en venir à faire" in the sense of "finir par faire" simply takes the form of Imparfait to describe a rather unlikely condition.
Q1. Given this, am I correct in assuming that the 1st is somewhat more unlikely than the 2nd?
Q2. You could never mix and match these two expressions and say "si jamais il venait à en venir à tenter ...", correct?