Hi and welcome.
I'll try and answer your questions.
1. Yes, she assumes the pm is still happening if lo needs it. As you push the am forward and forward, the pm becomes a catnap or quiet time. The trick is not to fight that last nap too much if lo won't take it, just offer it or offer quiet time in his crib (For me, the only way to get the catnap was a ride on the stroller or car, no longer than 25 mins). If pm nap doesn't happen, then offer an early bedtime to compensate lost sleep and to avoid overtiredness. DOn't fight the catnap too much or you'll end up with a lo overtired and reluctant to go to bed at night.
2. WTS should be the minimal intervention, and if you think a nappie change will wound him up, then wait if you can until he's up for the day.
If you need extra advice on the switching, then post what a typical day looks like for your lo and we'll try and help you with a plan. Good luck!!!!!