So far as how Hera could sense it too; at first, she couldn't at first. It took Jacen to even tell her it was there, and a clearly conscious effort to quiet her mind and open herself up to it.
I think people forget that Hera is every bit the Jedi philosophy convert as Sabine. She lived with Jedi for years, saw what they could do, even had a child by one. Indeed it was her idea for Kanan to take on Ezra as an apprentice, and it seems for Ahsoka to take on Sabine . . . twice! She doesn't need to unlearn what is possible, because she already has, many times over.
Couple that with her very strong connection with her naturally force sensitive son, and the fact that her innate piloting abilities may be part of a latent sensitivity in and of itself. Then I could see her sensing what Jacen feels through that connection; though it takes a conscious effort, and focus.
Yup, the balance of evidence is in favour of that really being Anakin's presence. Of course the distinction between this and "force dream" is rather meaningless considering that Anakin is a part of the force now. If it's the will of the force that in manifest a vision of Anakin Skywalker, why bother with a mere illusion when the real thing is right there?
Of course the whole thing is deliberately ambiguous, so we might never know 100% . . . but it's still clearly leaning pretty heavily in the "yes of course it's Anakin" direction.