Sightless Brawler or the creature it enchants can be declared as an attacker only if another creature is declared as an attacker at the same time.

If you control more than one creature that can't attack alone, they can both attack together, even if no other creatures attack.

Although Sightless Brawler or the creature it enchants can't attack alone, other attacking creatures don't have to attack the same player or planeswalker. For example, Sightless Brawler could attack an opponent and another creature could attack a planeswalker that opponent controls.

If a creature that can't attack alone also must attack if able, its controller must attack with it and another creature if able.

In a Two-Headed Giant game (or in another format using the shared team turns option), Sightless Brawler (or the creature it enchants) can attack with a creature controlled by your teammate, even if no other creatures you control attack.

On the stack, a spell with bestow is either a creature spell or an Aura spell. It's never both, although it's an enchantment spell in either case.

Unlike other Aura spells, an Aura spell with bestow isn't countered if its target is illegal as it begins to resolve. Rather, the effect making it an Aura spell ends, it loses enchant creature, it returns to being an enchantment creature spell, and it resolves and enters the battlefield as an enchantment creature.

Unlike other Auras, an Aura with bestow isn't put into its owner's graveyard if it becomes unattached. Rather, the effect making it an Aura ends, it loses enchant creature, and it remains on the battlefield as an enchantment creature. It can attack (and its {T} abilities can be activated, if it has any) on the turn it becomes unattached if it's been under your control continuously, even as an Aura, since your most recent turn began.

If a permanent with bestow enters the battlefield by any method other than being cast, it will be an enchantment creature. You can't choose to pay the bestow cost and have it become an Aura.

Auras attached to a creature don't become tapped when the creature becomes tapped. Except in some rare cases, an Aura with bestow remains untapped when it becomes unattached and becomes a creature.

You still control the Aura, even if it's enchanting a creature controlled by another player.

If the enchanted creature leaves the battlefield, the Aura stops being an Aura and remains on the battlefield. Control of that permanent doesn't change; you'll control the resulting enchantment creature.

Similarly, if you cast an Aura spell with bestow targeting a creature controlled by another player, and that creature is an illegal target when the spell tries to resolve, it will finish resolving as an enchantment creature spell. It will enter the battlefield under your control.