Answering your questions
a) DataJS can be used to connect directly to OData, but that is not the same as the Rest API method.
For DataJS see http://infocenter.sybase.com/help/index.jsp?topic=/com.sybase.infocenter.dc01920.0230/doc/html/vhu1345565629260.html
For RestAPI see http://infocenter.sybase.com/help/index.jsp?topic=/com.sybase.infocenter.dc01926.0230/doc/html/title.html
SUP is in proxy mode - Yes.
b) Yes
c) Relay server is a secure reverse proxy, SUP is an unsecure proxy. Use a Relay server or a Reverse proxy between the internet and your precious internal servers.
