Firstly, since the QMS is about product, then internal audits have no impact on product quality
Funny that standards used to model QMS's have so many requirements that are way above "product quality". Management reviews? we don't need any stinking management reviews....If QMS were truly about product only, we would still be in the early days of the Ford Model A assembly lines with no in-process inspection...assemble the whole thing and we check it in the end...
We know this to be true, because organizations which did good stuff for customers didn't do Internal Audit before they elected to do ISO.
No, we do NOT know that to be true. Using automotive quality as an example, we are light years ahead of where we were two decades ago. Gains have been possible with technological improvements, but also with quality system robustness. Internal audits, let's remember is one component of the CHECK step in the PDCA cycle. Now, if we are still debating the wisdom of PDCA, that's another story altogether.
Secondly, how someone is paid (W2 vs 1099) has no bearing on being "internal" in the context of the QMS and audits. Many contractors are used to perform product design, receiving inspection and they sit in the organization which contracted them to do the work. I'd wager that the contract was handled by HR, not procurement.
Let's not conflate employment terms ( full time, part time, short term, long term, etc....) with service providers. As offered by the OP, they want to outsource internal audit to an outside vendor. Nowhere it was mentioned that they want to employ someone part-time as an internal auditor. BTW, many aerospace CB's offer to perform first party audits for organizations they are not involved with, in terms of certification. Obviously CB's are accredited and that would answer one of the OP's first questions in this thread.
Thirdly, what you are proposing also would apply to training across a wide range of disciplines, especially since training helps make people competent and they affect product quality - so by extension you're advocating that
FMEA, Auditor,
8D etc training must be handled in exactly the same way? Odd that IATF which has the most stringent training requirements actually revised their SI to remove that aspect...
The difference here is that AS9100 has no requirements for training, per se. But it does have for internal audits, a mandatory process, which cannot be excluded from the QMS.