I'm still looking for an explanation. Say I'm an Independent on the fence and the explanation will do it for me. I recognize there are other flaws like suggesting a 2.8% unemployment rate at the end of the balancing project, but forget the other flaws for now. Just explain the Medicare part.
Let's say the plan is in effect 15 years from now. I'm a middle class or maybe slightly lower middle class person who has $200,000 in savings and have to live off of that and Social Security for the rest of my life and I'm 67. I find out I've got Lung Cancer. Average cost for a year of treatment, etc. is $275,000. How much is out of my pocket? Then it's in remission, average cost for a person in their late 60s, early 70s in that situation is $30,000 per year. I understand Ryan's plan gives an average of $7000 to each person in Medicare to buy their insurance. If most of that cost is not covered by Medicare, a person in that situation would be screwed. They wouldn't be able to afford health insurance. That's not rare. The two ways most people die in this country are cancer and heart disease. This will be a very common scenario.
If you're looking for a defense of the proposal from a supporter, you're unlikely to get anything other than platitudes and attacks on Democrats (they're taking their cues directly from Ryan himself on this one).
That said, the documents that have been publicly released about the Republican "plan" are vague and vacuous enough that it's impossible to answer your question with any certainty because it hinges on how insurers in the proposed Medicare exchange are allowed to behave. We know, as you pointed out, that a guaranteed issue rule is in effect so you can't be denied an insurance plan on the basis of having a pre-existing conditions.
According to the CBO analysis of what Ryan gave them, the proposal also sets up age rating bands so that insurers must charge everyone of a given age the same premium (though supposedly the value of the voucher "would vary with the health status of the beneficiary," which doesn't quite make sense if beneficiaries' premiums aren't determined by the risk associated with them but rather by the average risk of the rating band they fall into, but presumably that's just sloppy language; again, a detailed account of the proposal doesn't seem to be available).
So in your example, if you're 67 you'll get some set voucher amount--the voucher amount for a 65-year-old is apparently initially set at $8,000 (on average), and seems to tick up in some not-quite-defined way as you move up through the age rating bands. So maybe as a 67-year-old you can get $9,000. You then take that money into the Medicare exchange and look for an insurance plan. It's taken as a given that this won't cover even a majority of your premium (CBO pegs the average voucher as covering 39% of an insurance plan comparable to the current Medicare benefit in 2022) and that the value of the voucher won't grow as fast as your premiums.
The first question is what plan offerings look like and how exactly the rating rules work--are they relatively standardized to allow meaningful choice between competing options and must an insurer charge everyone of a certain age in its pool the same premium, or merely everyone of a given age choosing a given plan? That is, how much power do insurers have to segment risk, shuffling off the least healthy beneficiaries into a given insurance plan (i.e. the one with the treatment they need) and jacking up premiums to discourage enrollment--or perhaps just to bleed the beneficiary dry.
Once you've nailed down how premiums and plan/benefit offerings work, you need to figure out what restrictions the GOP plan puts on cost-sharing , annual/lifetime limits, etc. That is, how much of their costs can they put onto the shoulders of beneficiaries. I suspect there would be few or no restrictions on that--or most insurer practices, for that matter.
So, given the little information we have, it's not possible to say exactly what share of that figure you quote you would have to pay, other than "substantially more than you would under actual Medicare." The answer to your question of "who foots the bill" is simple: you do. Shifting costs to beneficiaries is in fact the only way the GOP proposal "saves" money (i.e. saves the federal government money--there is no serious attempt to address the actual cost of care). However, regarding some of the language you used, note that under the GOP proposal,
no health care cost is covered by Medicare because Medicare no longer pays for the physician and hospital services of the elderly. Medicare as an insurer--a payer for seniors' health care consumption--would no longer exist.