> What's the corroborating evidence from ~750BC documenting their existence? Heck, where's that evidence from 650BC?

This is a totally unfruitful line of inquiry, unless you are an academic historian doomed to publish-or-perish; in that case, it's perfectly understandable that you have to make these arguments as part of your career.

Better questions, when it comes to ancient history are:

1. Is there any negative evidence? 2. Is there any reason to expect more evidence than what we have? 3. Did ancient historians with better sources believe it? 4. Is there anything inherently unlikely about the idea?

In this case there isn't any negative evidence, there's no reason to expect more evidence than what we have, ancient historians believed various versions of the same basic story, and there's absolutely nothing crazy about it - someone had to found the polity, and to declare them fictional results in there still existing a first king of Rome, just a guy with a different name for whom there is no evidence at all. You can certainly apply (4) to specific details of the different stories - for example, it seems much more likely Romulus was murdered by Senators than raptured by Jupiter, though perhaps something altogether different happened. There's even very weak archeological evidence - a chamber was found in an area where Romulus' (empty) tomb was supposed to be, although archeologists are debating over whether it really is that.