Hi,
We can confirm that it is possible to post a supplierInvoice with a currency that is not specified on the supplier independantly on the settings on the supplier and have reported this to the development team.
We will let you know when progress in this case has been made.
Regarding your second point, we have created a new idea and forwarded this to the development team, progress can be followed in this post: Expose "currencyOverride" & "currencyRateOverride" for GET Supplier
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