i2c error seems related to (I guess) a RGB controller that is presumably controlled by something else (or maybe not usable/non plugged). But it's harmless unless you are looking to configure RGB (but I don't think you want to do that anyway).
EFI_MEMMAP warning is probably because Dom0 doesn't see UEFI mappings, as it relies on a different method to do UEFI calls. This is expected and this warning doesn't indicate a problem.
The latest error is likely related to the first one.
what I will miss /won't be able to do vs a user that has not these messages? (otherwise, why they would be raised)
Nothing meaningful.
if these messages /failures have an impact on the time during which the machine is booting? (it seems the load process is hanging for about a minute)
No.
if these messages /failures are officially documented somewhere?
For the EFI_MEMMAP, XenServer has a article on it stating the same as me : https://support.citrix.com/external/article/CTX331542/citrix-hypervisor-82-efi-efimemmap-is-n.html
how can I resolve these failures, because failures are failures... even if "one should not worry about them"?
You may be able to hide it, like by blacklisting i2c and such, but TBF it's not worth the time.
