I am new here, so I hope I posted this correctly! If you live in a no fault state, what is/are the benefits of proving infidelity prior to separation? I am debating on hiring a P. investigator, but if it won't matter, why waste the money?

There is normally no need or reason, also you can in the process of the divorce send them a questionere to answer , and you would be surprised how truthful they are when it is a court document ( sometimes)

But if there is a child custody issue or visitation issue, some matters like drug use or immoral behavior may be good to have, but for a standard divorce it is normally thowing good money away.

No fault means just that ' "no fault." That is to say, legally nobody's at fault. From a practical or moral standpoint that may not be the case but the law isn't concerned with that. That said, I wouldn't waste the money on a PI.

What if there are suport/custody/visitation issues for a minor child, and 3 older children who will be emotionally distraught over the situation. I have been made to look like the controlling/psycho parent for years by spouse, when it is, in my opinion, his emotional issues/weaknesses at the root of most of the problems. I somehow feel like I will be made to appear as the reason why it didn't work out when in reality it has been me making the majority of the effort to resolve marital issues. Somehow that just doesn't seem fair! I don't want my older children blaming me.

Nobody said the system was fair or even relied on common sense. The rights of the parents, both parents, trump all else in these situations. The character of the parties involved or the circumstances which led to the breakup of your marriage are inconsequential from a legal standpoint.