PC frequently doesn’t produce crescent scars on peaches (though sometimes they do). They almost always produce scars on plums/cherries. You can identify a PC grub pretty easily because it’s legless vs. OFM and CM which have some legs.
Codling moth don’t attack stone fruit and don’t cause flagging. OFM attack primarily stone fruit but will also attack pome, particularly where both kinds of fruit are grown together. OFM also attack the shoots.
As Cityman pointed out, OFM pressure can be high causing tree damage. My advice would be for now, just to spray the fruit, since you have so few fruit to protect. Then watch for flagging. A tree will tolerate a little flagging and creates a little bushier tree. If it gets excessive, as it did in Cityman’s case, you’ll just have to spray the whole trees. Otherwise a little quart spray bottle would be fine.
Be aware that Imidan breaks down quickly in alkaline water, so you’ll probably need to acidify the water, if you want to keep the spray you don’t use. It won’t last forever in acidified water, but will last a lot longer.
Imidan has good residual protection on the tree. It retains an insect lethal residue for a couple weeks unless rain has washed it off.
I respect the credentials of your Ph.D entomologist friend, but unless he is also a fruit tree specialist, I think he’s a little bit out of his area of expertise (even if he is a fruit tree specialist, I think he’s being a little OCD about insecticide rotation). Certainly most labels of commercial insecticides recommend insecticide class rotation, but I’m not aware of a single home owner fruit tree insecticide which does.
Tippy is correct that rotation is meant for commercial orchards. There is a vast difference between a commercial peach grower like Titan Farms with 5000 acres of peaches and a backyard grower with 5 peach trees. The risk of the backyard grower producing a resistant strain of insect by way of spraying practices is pretty much non-existent. The numbers just aren’t there.
Additionally, most rotation programs don’t advise rotation with every spray. The current thought is to rotate insecticides with each new generation of insects, monitored via IPM practices like trapping, degree day calculations, etc.
There is perhaps one advantage of rotating insecticides for a backyard grower. That is, if there are some resistant insect pests in the area, a rotation could provide better control over using one insecticide, if that insecticide happens to be the compound which the insect has resistance to.