I am afraid that the Spreadsheet does not allow changing the Column headers in the requested approach. You can however, use the row after the A, B, C... in order to set custom Header, as in the following dojo:
I am afraid, however, that the second and third requests are not supported with the Spreadsheet. You can use the portal, in order to submit a feature request though:
Please let me know, should you have any further questions.
Telerik by Progress
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Feedback Portal
and vote to affect the priority of the items