There has been lots of talk about bridging healthcare after retirement before Medicare on this forum.
Here is one thread
The stipend will be nice as it will cover the premium, but it will count as taxable income. That will most likely eliminate the possibility of having your premiums supplemented by the government.
Yes, the ACA programs vary a lot from state to state. In the states I am familiar with, they only cover medical care within the state you live in (except for emergencies occurring out of state).
We have used exchange health insurance since my husbands early retirement in 2020 in two states now - MA and NJ. The original process of getting the coverage was not bad. My husband as of 9/1 is on Medicare (turns 65 this month). I am a year younger. It took us over 4 hours of phone conversations over at least 6 phone calls over about 2 weeks to get our coverage correct. All we wanted was to end his health coverage and keep my health coverage and both our dental coverage.