That will be a problem no matter what.
It usually don’t
That depends - I can reinstall my system in 5-10 minutes
Yes
That depends on your current partition schema
Don’t - use the ventoy project instead
Just reboot and use your systems boot override - e.g. F7 or F12
That dependes on your available diskspace - but EndeavourOS can run on as little as 10G and 1GB RAM - depending on the DE installed.
The reason you are advised not to format it is obvious - you will prevent Windows from booting. Watch out - if you are doing a manual partitioning but as Calamares usually default to not formatting existing partitions - you should be safe.
What can you imagine?