Moss, you're not thinking clearly. You're never been through the full affair process. Most here, if not all, have.
Things, concepts, assumptions you need to put aside:
First, faulty concept your pre-affair wife is the same person as your WW. This concept causes more trouble, more pain/heartache than anything else.
"My WW will tell the truth and will understand logical reasoning." Sadly no she won't, she'll answer questions based on what she THINKS you know, and will promise/ agree to anything you say if you stop this "you need to stop seeing him" line of thinking.
Analogy: her mindset is closer to an alcoholic or drug addict.
Now maybe this is still in the EA phase, it can escalate in a heartbeat. Review the term "limerence". It's easier to break the "love addiction" at the EA phase, once it turns PA, it becomes extremely difficult to break the A.
PA phase, Most likely she will become cold&distant and/or argumentative.
You have to get ahead of this, I can't stress this enough.
My belief is you came here because at some point you felt something wasn't right. Don't discuss this with her, tipping your hand will cost you dearly.
Let's say you honestly go through with your plan. You may get lucky "she sees the error in her ways" and everything returns to normal. It would be nice, but not likely.
OR, she hands you her phone, nice maybe nothing is there, because she deleted the damning stuff.
So you escalate, telling her you're going to do a phone recovery for deleted items.
Now she gets pissed, starts grabbing for the phone, starts screaming at you about not trusting her. Maybe she calls 911, the cops show up, you're arrested for DV.
If you just do a copy of the phone, you avoid this.
Btw, she may have started using a burner phone.
The big piece is the whatif is like a cancer. It can emotionally eat you alive. If she resets her phone, you'll never know what she was hiding.
Loss of trust is the biggest part of affairs, it destroys marriages. How will you feel about her destroying evidence on a phone?
There maybe nothing or maybe there's more evidence of a solid EA. Whatever you find, you'll have firmer ground on what you'll need to do.
You'll be moving forward, right now you're standing still. Most of us in the beginning are more concerned about doing the wrong thing, thereby making it worse.
If she's in the beginning of an affair, doing nothing means you're helping the affair. The more time you let pass, means that much more opportunity for it to get worse.