Two Health Insurance Plans. Help!

Thank you and sending all positive vibes to you in advance for reading and answering my q’s.

I am on my parents health insurance plan and also have the option to purchase my works health insurance plan. I am considering purchasing the work plan because my parents plan, although phenomenal, is an HMO plan. I have complex health conditions that require several specialists. The HMO network has these specialists (for example an Immunologist) but not ones that specialize in my condition. With the HMO, in network visits are always a $25 co pay.

The work plan is an employer funded PPO plan, with a monthly premium of $151, a $1600 deductible and a $3600 out of pocket maximum. There is also an HSA associated with the plan, but I was informed that if I am also on my parents insurance I will not be able to contribute to the HSA. My work also offers an FSA but the FSA can not be used for medical expenses when you have an insurance plan with an HSA.

The PPO plan uses Express Scripts as a PBM and the HMO plan uses Prime Therapeutics. I have cross checked all my current prescriptions against both formularies and they are both covered, BUT, the PPO plan tiers my prescription. There is also a difference between the cost of the medication, $25 with the HMO and $75 with the PPO.

How does coordination of benefits work when it comes to prescriptions? Am I stuck with the PBM that is associated with my primary insurance?

If I see a doctor that is in network with both the HMO and PPO, but I have not met the $1600 deductible for the PPO, would the secondary HMO insurance cover the difference between the $25 co pay and the cost of the appointment, and would that difference count towards my PPO deductible?

Is the inability to contribute to an HSA something specific to my employer, or is this a common rule?

Am I better off not signing up for insurance though my employer and contributing to an FSA, and hoping the HMO will cover out of network doctors via single case agreements?

Thank you for reading this far, if you have any other useful advice or information that I have not asked about, please feel free to share. THANK YOU AGAIN.