I think the relevant document is IRS Pub 969, in the section "Distributions from an HSA" several subheadings down:
Additional Tax There is an additional 10% tax on the part of your distributions not used for qualified medical expenses.
which is followed by:
Exceptions. There is no additional tax on distributions made after the date you are disabled, reach age 65, or die.
So your understanding is mostly correct -- the age threshold is 65, not 59.5. Under current rules. The rules could change between now and whenever you reach 65. With that said, I'm mostly just leaving money in my HSA -- that account has the highest rate of return of any (insured) account I have, and like you say it is basically another way to get a deduction and save for later.
I couldn't find any info on rolling over to a Roth or an IRA, except that you can't do it. (Though none of the sources I found mention a reference to IRS docs.)
Edit: I should also mention that, after 65, distributions from the HSA to pay insurance premiums are qualified (tax free). See the link to Pub 969 above. So if you are careful about how you spend the HSA money in retirement, it can be used tax free. (I.e. spend out of the HSA to cover medical expenses, then draw down your IRAs for non-qualified spending.)