Yes, that is the correct possessive form of the singular noun toothbrush. example: Don't buy that one, the toothbrush's cover is cracked.
The noun form for the adjective horrible is horribleness.
The noun form of the adjective obedient is obedience.
The noun form of the adjective 'prosperous' is prosperousness.A related noun form is prosperity.
The word bought is a past tense verb, not a noun. There is no plural.
The noun form of "punish" is "punishment."
The noun form of "religious" is "religion."
The noun form is the act of demanding, or something that is demanded, the word you need in 'demander
The noun form of "acrimonious" is "acrimoniousness."
The noun form for the adjective glaring is glaringness. Another noun form is glare.
The noun form for the adjective austere is austereness. Another noun form is austerity.
Implication is the noun form of "imply."