Hi diydog,
I really dont think modifying the A record will let you use the domain as an addon in the CPanel. Cpanel need a domain to be pointed to it directly, like the authority should rest with heliohost in this case. So you must change the 1and1 name server and setup the email mx records on the heliohost server. that is the only way cpanel will let you use the domain as an addon. There is no work around unless you want to try the manual A and NS modifications . I dont know much about how that is done but this is what I know. I tried the same and believe me it didnt work.
Jose