While the guidance was provided back in 2012, nothing seems to have changed about it.

"It seems like by now they should have some guidance because it looks to me like people have been getting these rebates for several years now."

The rebate going to the insurance policy holder hasn't changed since that guidance was issued, because the rebate provision requires that it goes to the policy owner (not who pays). Yes, it even has been known to exceed the policy premiums paid. That has only changed in these years, in that the insurers have been able to get closer to the "80/20" requirement, so less is going out as rebates.

The scenario seems to sum up as:

The rebate is like a penalty to the insurers and it has to be returned to those who were impacted by this lack of responsible cost management (not the same as who paid). That's why it doesn't go back to the Treasury.

If you deducted the premium through one of the available options, the rebate is then taxable income to you for the year you got that rebate, the same as other tax regulation applies for deductions that you take and then later you get the money returned, afterall.

There would be nothing to amend for the year of the premiums and this has nothing to do with the subsidy. Getting a rebate in 2020 for coverage in year 2019 simply requires you to examine if there is a credit or deduction for it in 2019. If so, it is taxable in 2020. If not, it also is not taxable in 2020. It does not change 2019, so no amendment.

The qualification for the subsidy is based on the table for poverty levels and on the income computation. The Rebate is not used to compute against the Premium; they are not part of that computation for the year of the rebate nor the year you get the rebate. If it is part of income in the year it was issued, it affects if there is qualification for a subsidy for that same new year.

I've never seen anything that states an insurance rebate gets returned to the Treasury.