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According to Moen, this model does have some plastic pieces but the part you asked about is not plastic.
Yes, this faucet is a one hold installation. This is a pull down sprayer, so the head pulls away from the body of the faucet and you can use that as a sprayer.
According to Moen, you do not have to hold the button to keep a spray mode. If you click the button once it will switch modes.