Has this ever really happened? Seminaries and Religious Orders have very rigorous selection procedures and novitiates in order to weed out those who are entering the priesthood for the wrong reasons.
Any priest who had this approach would be unlikely to succeed, as he would have to obey his bishop, otherwise there would be consequences. He would need to be a Catholic for at least 3 years before entering novitiate, and show a genuine enthusiasm for serving the Church. It would be far from certain that any priest would ever become a bishop, and I think the selection process for the episcopal office would be even more rigorous in its’ selection than that for priests. Even as a bishop, he wouldn’t really be able to bring the Church down from the inside. He couldn’t teach heresy, because Rome would stop him, he couldn’t destroy the finances of the Church, because his own diocesan board of trustees and diaconate would stop him. He could create scandal, certainly, but the Church has survived scandal before. It would be an extremely hard (and pointless) path to follow, and I honestly don’t believe that anybody can be a priest in their own strength, without the grace of God.
You have to distinguish between those priests who may be in error, or may, due to their own pride, bring false opinions into the Church with good intentions, and those who are genuinely out to deliberately destroy the Church. I really don’t believe there are many (any?) priests in the latter camp.
Also, is it really possible to fool the Holy Spirit, who protects the hierarchy of the Church? If there are such priests and bishops, they are there because God has some kind of purpose for them. I don’t believe that anybody could live as a candidate for seminary, a novice, seminarian, then as an obedient priest for many years without the Lord beginning to have an influence on their lives. By the time they became a bishop, maybe their whole motivation would have changed.
Just as we can receive the sacraments in bad faith, but they are still sacraments, so they remain sacraments if they are offered in bad faith. A priest can still say a valid mass even if he is in a state of mortal sin.
If I were you, I wouldn’t lose any sleep over it.