The key word here is indeed beast.
Beast is a defined type of creature within the game, as per page 6 of the Monster Manual;
Beasts are nonhumanoid creatures that are a natural part of the fantasy ecology. Some of them have magical powers, but most are unintelligent and lack any society or language. Beasts include all varieties of ordinary animals, dinosaurs, and giant versions of animals.
Since Wild Shape specifically says
assume the shape of a beast
you are limited to only creatures that have the beast type.
Note that at level 10 a druid of the Circle of the Moon gains the ability to Wild Shape into an air, earth, fire, or water elemental, but it specifically limits it to those 4. This does not allow the druid to Wild Shape into any creature with the Elemental type (such as a Galeb Duhr or Gargoyle).
It's not specified.
D&D 5th edition's Mordenkainen's Tome of Foes is the first sourcebook to reveal that the Ruby Rod was forced upon Asmodeus by Primus, and that devils obey their contracts out of fear of the rod's curse. It's not specified in any other 5th edition book, and the relevant sourcebooks of earlier editions make no mention of this curse either.
D&D 3rd edition's Fiendish Codex II: Tyrants of the Nine Hells defines Asmodeus, the Ruby Rod, and the rules around mortals making a pact with a creature. On page 23, regarding Faustian Pacts:
Though benefits of extraordinary value can be offered in a Faustian pact, the devil preparing it always tries to achieve the deal with a minimum possible expenditure of resources. Once the soul is securely damned, the negotiator often arranges—usually through servitors—to bring about the signatory's untimely death.
Page 25 describes what happens if a soul goes to Baator and demands the right to adjudication on the grounds that their rewards were not granted, or that the mortal was coerced or magically compelled:
The judge, usually a pit fiend, listens dispassionately to both sides and rules, as a lawful creature must, according to the law.
If the mortal wins, their soul goes free and may be restored to life with raise dead as normal. Notably, there's no mention of any curse or penalty which affects the devil who made the pact.
Asmodeus and his Ruby Rod are detailed in this book (p.155-157). There's no mention here of the rod's origin with Primus.
In AD&D 2nd edition's Planes of Law, it's suggested that devils obey their contracts because they are experts at creating and exploiting loopholes in contracts, so the contract is always in the devil's favour (p.20):
They'll cut a deal with any berk they can, and woe to the sod who agrees to their terms — and dire woe to any sod who thinks he can skirt the agreement! The baatezu've been making deals since before most Prime worlds cooled from the heat of creation; they've learned just about all the loopholes.
Best Answer
DnD 3.0:
Book of Vile Darkness (121-122):
A select choice of abilities of the Ruby Rod is usable at will by Asmodeus, but only once by anyone else. The wielder can cast Bodak Birth as if cast by a 20th-level spellcaster.
Bodak Birth (Book of Vile Darkness 86)
Demand Submission (Book of Vile Darkness 165) is an ability by Asmodeus, not tied to the ruby rod:
ADnD 2e: Guide to Hell (51)
DnD 3.5: Fiendish Codex II: Tyrants of the Nine Hells (156):
The Aura of Submission is notably an ability of Asmodeus, not of the ruby rod:
DnD 5e DMG (64)
The Nine Hells:
DMG (66):
Mordenkainen's Tome of Foes (9-10):
The ruby rod was given to Asmodeus by Primus of the modrons as part of the Blood war trial issued by the angels.
Conclusions
Editions in brackets after segments.
In the mentioned editions the ruby rod is a sign of office, but the power that creates submission is based on the order of hierarchy in the hells (2;3.0;3.5;5) and manifests in Asmodeus, not his ruby rod (2;3.0;3.5). While in 3.0 you can create a Bodak that has to obey unless it saves and then risks disintegration (3.0), there is no magical enforcement outside of that obedience and the obedience created by the ruby rod as a sign of the ruler of the hells (2;3.0;3.5;5). All Further suggestion or geas that can't be resisted are tied to Asmodeus (3.0;3.5).
A wielder can use the ruby rod and will receive obedience by any devils (2;3.0;3.5;5), but the ruby rod does not grant the power to create magical pacts that enforce the law (2;3.0;3.5;5). The wielder of the ruby rod is regarded as the ruler of the hells with its natural law and hierarchy (2;3.0;3.5;5).
The law of the hells is enforced by superiority, and thusly obedience is granted. Such is the law of the Nine Hells (2;3.0;3.5;5).
While the ruby rod is a sign of the office and any devils will obey the wielder, Asmodeus is still the undisputed ruler (2;3.0;3.5;5) who may grant the ruby rod to someone lower than himself, under his law of superiority (2;3.0;3.5;5).
And sometimes Asmodeus bestows his ruby rod onto a mortal to serve his ends (2;3.0;3.5). That mortal will see the obedience of devils (2;3.0;3.5;5).
The ruby rod itself can only punish the devils who breach their contracts with mortals (5). It grants them the right by the angels' trial, not the power (5).
The ruby rod grants Asmodeus and his underlings the right to enter into contracts with mortals, that his underlings have to be devils is not explicit. Whether the rod can extend its enforcement to none devils is also unclear (5). The laws of the hells regulate all devils and even Asmodeus (5).