The power-drain inator stole Mjolnir's powers.
After the Marvel heroes were hit by the power-drain inator, Thor still holds Mjolnir in spite of being powerless since, as he later explains to Iron Man, it's not a matter of power but of worthiness. However, after Thor throws Mjolnir, the hammer just falls down instead of going the way Thor intended to. It happened not because Thor became powerless but because Mjolnir became powerless. When Thor got his powers back, he got Mjolnir's as well but the two are so bonded that it allowed Thor to summon it and unknowingly transfer the powers back. Mjolnir only stayed where it was during the time Thor didn't have his powers was because it was too heavy to be lifted by powerless individuals, worthy or not. Baljeet passed through The Green Door
Baljeet...like all others who have been touched by gamma and been made into hulks, died and passed through the green door when cursed with hulk powers. Now, he is unable to die. Phineas and Ferb would have built a substitute Mjolnir if they worked under the above guess.
Justin Hammer's adaptoid has already proven it possible. Unable or unwilling to work on the concept of "worthiness" since their own creators had to be told Iron Man couldn't be given it, Phineas and Ferb would set Mjolnir-2 to work for whoever got the powers they designed to replace Thor's, meaning Iron Man would be able to wield it. Mjolnir was no longer at the place it was left behind when Thor got his powers back.
To prevent more fines from being issued, Nick Fury asked Captain America to pick up the hammer since Cap, unlike Tony, is worthy. Cap was in the middle of delivering Mjolnir and the fines to Thor in Danville and joining the battle against his archenemy when Thor regained his powers. Captain America was fighting Baron Zemo during the episode.
This episode's version of Venom is Eddie Brock.
The royalties Spider Man gets for having his image used in the comics that exist in-universe are being used to pay for damages he's blamed for.
That's Peter's usual luck regarding money. Mission Marvel takes place post-Civil War
Which is why Spider-Man is there and Captain America isn't. Cap died prior to Mission Marvel, and Spider-Man joined in his place