I don't get your point.
1.) The fully implemented kext should contain all HS/SS ports necessary to fully implement all onboard USB2.0 and USB3.0 connectors/headers of the respective motherboard. Following your own port-layout.rtf, on your motherboard only 18 HS/SS ports are required to achieve the latter, with SSP6 being the highest SS port to be implemented. There are no further USB3.0 connectors to which SS07 to SS10 could be assigned. BTW.. for USB2.0 backwards compatibility, also HSxx ports should be assigned to such USB3.0 connectors. If you still claim something contrary than currently implemented, proof and demonstrate it and update your port-layout.rtf, respectively.
2.) I do not see why IONameMatch XHCI should not work in your case. Anyway, if the latter would be really the case, it would be rather because of SMBIOS iMac18,3 or some inappropriate ACPI replacement in your config.plist than because of your specific motherboard.
3.) I thought it was obvious why one also needs the fully implemented kext.
a.) The fully implemented kexts (18-ports in your case) still can be truncated to 15-ports up to each users personal taste and requirements, moreover as there is also the complete port-layout.rtf with the proper assignment of all HS/SS ports implemented. It appears to be the easier approach than modifying an already truncated 15-port kext subsequently.
b.) There are people who insist in using the USB port limit patch under macOS version different from 10.14.1 or 10.14.2. They insist in using all onboard USB2.0/USB3.0 onboard connectors/headers against all warnings of likely induced buffer overruns or all recommendations against loosing vanilla system compatibility. To avoid all unpleasant respective discussions, an additional disposal of fully implemented kexts seems also unavoidable, simply because of strategical reasons.
c.) In contrary, the already truncated 15-port XHC USB kext basically should serve all noobs having no clue on the entire matter, i.e. a predefined OoB solution independent from the employed macOS version.