Hope all is well.
We are validating the CIM Hosted page scenario.
It works fine but we have few questions regarding UI Display:
To address your questions:
1) Note that it is not possible to hide the billing and shipping information. You can use the addPayment or editPayment form to show billing only and not the shipping information.
2) You cannot add custom fields using the hosted form.
3) The iframecommunicator can be used to know when the customer saves or cancels the form. The profile ID used to open the form can then be queried to find what was changed.
4) This is not possible.
5)There is no relay, but you can set the "hostedProfileReturnURL".