"Less efficient" is actually a nonsensical term to economists[0], but monopsonies are inherently inefficient, and it's rather straightforward to demonstrate that economically, the same way it's straightforward to demonstrate that a monopoly is always inefficient, even in cases of so-called "natural monopolies".

> Are you familiar with the impact Medicare Part D had on prescription drug prices? It "substantially lowered the average price and increased the total utilization of prescription drugs by Medicare recipients

You can't use Medicare Part D as an example of the impact that negotiating power would or would not have, because Medicare doesn't "negotiate" (read: unilaterally determine by fiat) the prices it pays for drugs, which it does do for providers.

> Are you familiar with the many estimates of the cost of a single payer program in the US? [2]

Yes, and most of those (including the ones you linked) are based on Medicare's reimbursements as a baseline. But that's totally baseless, because according to Medicare's own financial reports, they pay below-sustainable reimbursement rates to providers[1], which means that they are essentially subsidized by the premiums paid by privately insured patients. If Medicare were expanded to include everyone, they would have to massively increase their reimbursement rates, because the private insurance market wouldn't exist.

[0] Efficiency is best explained to a non-economist as a binary property; there does not exist a total (or even partial) ordering of inefficient states.

[1] They pay below COGS, which means that even if all doctors, nurses, and staff worked for free, they would still make a loss on Medicare patients in the aggregate, if they were not able to charge the difference to private insurers. Incidentally, that's why uninsured patients see such high bills - those are the prices that are used as a starting point when negotiating with insurers, and they're massively inflated to cover the gap.