A1. You can't. Marketo fields can be hidden, but not deleted.
A2. Probably a good idea to get your Dynamics admin on this (unless that's also you? )
On top of Sanford's answers,
For Q1: Before hiding the fields in Marketo, go to Admin -> Microsoft Dynamics and click the "Edit" button, then unflag the fields you do not want to use anymore so that they no longer sync. Then, nnce you have hidden the field (adminb -> field management, select the field, the field actions -> hide field), you can llog a support ticket to have the fields "deep hidden", which will remove them from the user interface.
For Q2: Once the fields are created in Dynamics, back to Marketo and go to admin -> Microsoft Dynamics. There, you will be able to edit the list of synchronized fields and check these new fields for sync.