Question

Does Multitype also prevent item loss?

Answer 1

No because it will change that form until the battle ends or it loses the item. Just like Cherrim with the sun, When the sun is out Cherrim changes forms but when the sun is not out or ends it changes back. Just like Arceus with a fighting plate. It turns into a fighting type until the battle ends or it loses the plate then it will be normal if it loses the plate.

Answer 2

Yes in Generation IV, but not in any other games.

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In Generation IV:

The item-removing effects of Thief, Covet, Trick, Switcheroo, and Knock Off will not work on a Pokémon with Multitype, regardless of what item it is holding.

From Generation V onwards:

Multitype no longer has an effect on items; instead, item-manipulating effects cannot affect Arceus if they would cause it to hold a Plate or take a Plate it already holds.

Multitype (Bulbapedia)