#1 Typically does someone (i.e. the HDHP insurance company or administrator) "negotiate" discounts from the healthcare providers?

You will present your insurance card to the healthcare provider just like any other insurance. It will be billed through your provider, rates adjusted, and then the total balance sent to you to be paid.

#2 Are there any restrictions or tax consequences for a person joining Medicare when they reach age 65 if they previously had a HSA?

From: http://www.treas.gov/offices/public-affairs/hsa/faq_eligibility.shtml

"You are not eligible for an HSA after you have enrolled in Medicare. If you had an HSA before you enrolled in Medicare, you can keep it. However, you cannot continue to make contributions to an HSA after you enroll in Medicare."

Hope this helps!
- Paul