Occam’s Razor suggests they are not the same person, really. For one, if Mara was the same thing as Beaumont, it would mean Lilith was either in on it from the early days of Solomon Island as it is now (around 1700 or so) or she was unaware of it. And given she’s older than who Beaumont is supposed to be…
Not to mention, to address the connection to the Black Pharaoh, that the sun referenced is the one we see in CotSG. The one whose nature we fully know. And, so far, we’ve failed to see any Dreamer or Dreamer-related thing be half as human-looking as either Beaumont or Mara.
Also, remember: what we know is associated with Loki, may just be memories that became legends, that faded to myths. He may not be able to shapeshift anymore. He may not never have been able to shapeshift. If Beaumont is Loki, which seems very likely, it’s probably something along the lines of Lilith: someone somehow bypassing the flipping of Ages, becoming yet another reflection of themselves. No shapeshifting involved.
As for Mara… I’d think her husband would have reacted to her not being her when he staked her at the end of the Transylvania storyline.