1- There are hadiths that state Imam Ali’s (a) spirit would aid previous prophets and support them. According to these hadiths, Allah would support His prophets by sending Imam Ali’s (s) spirit to aid them.
2- Yes, the Jinn would also help some prophets. In the case of Bilqis’s throne, it wasn’t a Jinn who brought it. He offered to bring it, but Asif ibn Barkhiya (the successor of Prophet Sulayman) ended up bringing it within a blink of an eye. He had some knowledge of the “book,” so he had this supernatural power.