Whether to say "en plein préparatifs" or "en pleins préparatifs" with accord?

Question

Vous êtes en plein préparatifs pour décorer et organiser la chambre du bébé à venir?

or: Vous êtes en pleins préparatifs pour décorer et organiser la chambre du bébé à venir?

Given that the noun "préparatifs" is plural masculine, I'm inclined to plump for "pleins".

But then again, when the phrase "en plein" (as in, "en pleine rue" or "en plein jour") has the meaning of "in the middle of something", should we always treat the noun as a singular entity, thereby invariably coupling it with a singular adjective "plein(e)", even if the noun is in the plural like "préparatifs"? So is the apparently mismatched "en plein préparatifs" actually the correct form?

On several separate occasions, I have seen French speakers use "en plein préparatifs" in email.

Answer 1

Plein is a tricky word that can be a preposition, an adverb, an adjective or a substantive.

It is however clearly used as an adjective here so there is no reason for it not to agree with the name it applies to:

En pleins préparatifs is the correct form and the singular plein is a mistake in that position.

See http://alafortunedumot.blogs.lavoixdunord.fr/tag/en+plein

This mistake looks to be quite common and can be found even in printed books.