let me first clarify that i am no expert on this, and admin who do this for a living and have to be aware of things like this likely know better than us, especially me.
if they're saying it's possible, my suggestion would be to call financial aid and confirm with them. if they say starting before july 1 will result in loans being available as they are now, then okay. i would try and call someone from the other side (government) to verify with them. if all checks out i think it's reasonable to assume that it might be okay.
my only concern is that new rates and all that stuff refreshes july 1. even if you're starting before then, you need to have grad plus loans taken out BEFORE then in order to qualify for continual use of them. same applies to class of 2029. say they pay for their entire year without taking loans as an M1, they wouldn't be able to take out grad plus loans for M2 since even though they started their program prior to the end date, they didn't take out a loan. so just be sure that you can get a grad plus loan out before july 1, 2026, or else it won't really matter when you start.