This question seems pretty specific to the software you are using, and based on prior posts you’re using carveco, most users of this forum are not carveco users and the guidance received will not alway be accurate for your uses…
You can get additional carveco related guidance over in their Facebook group or discord.
Now to address the questions you’ve asked…
What toolparth type is the roughing and finishing question pertaining to?
If its 3d then finishing stepdown isn’t used, and multiple z passes should be turned off…
The amount left after roughing is controlled by the allowance setting.
Will that effect finishing speed? Not unless you slow down the settings in the finishing section.
no, this is the multiple Z passes option, which you would want to use for ROUGHING only… For Finishing passes the only reasonable use I have found to actually turn it on is when carving a softer metal like brass, so if you are carving woods do not used the multiple Z passes option for the FINISHING toolpath.