1、 If oVirt can be used to manage RHEV ?
RHEV has been rebranded several years ago to RHV.
If you already have RHV I don't see why you would want to use oVirt to manage it instead of using RHV-M (downstream of ovirt-engine) but from a technical perspective it should work just fine.
I am not sure what exactly the OP had in mind, but generally speaking, I'd recommend to _not_ mix RHV and oVirt. Although they are very similar, there are small differences that might bite you. A recent example I ran into myself recently:
I tried to restore on oVirt 4.4 an engine-backup taken on RHV (4.3, but it doesn't matter):
RHV uses a different version of wildfly (JBOSS), from a different location, and it does that by including a conf file pointing at it. If you take a backup, it includes this conf file, so it's also restored on restore. So when you then try to start the engine this fails because it can't find wildfly in the configured location.
Best regards,