Nora (a.k.a. Furuhime) retains the mark of her past masters who never officially set her free, so shouldn't she still have Yato's mark on her body, allowing him to summon her at any moment?
If he can, is there any good in-universe reason that he didn't summon her during the final battle (in episode 11) in order to disarm Rabo? Couldn't Yato have summoned Nora, dropped her onto the ground, and then used Sekki (Yukine) to deal with a weaponless Rabo?
Bishamon is able to have multiple shinki summoned into their non-human form at once, so there is no restriction that you can only have 1 shinki in weapon form at a time.
I assume that once she has been summoned by a god, she cannot be summoned by another god.
Also, while she was in her human form in episode 11, up until Rabo summoned her into his weapon version of her (Reiki), Yato could just as easily have summoned her into his weapon version of her, and if he had done so, Rabo could no longer summon her.
Is this a plot hole: that Yato just didn't think of disarming his opponent when he easily could have done so?