Yes, decidely is an adverb. It modifies an adjective and means to a definite extent or degree.
decidedly
Decide is the verb and decidedly would be an adverb. But it has other connotations besides referring to a decision.
It is decidedly so
No. Decision is a noun. The adjective decisive has an adverb form, which is decisively, and the past paticipe adjective decided (substantial) has an adverb form decidedly.
most decidedly not most decidedly not
it is decidedly so.
Although I liked both cars the smaller, more economical one was the decidedly better choice.
Decidedly Bloody Dodgy - 2004 is rated/received certificates of: UK:15
Decidedly eukaryotic.
The cast of Decidedly Bloody Dodgy - 2004 includes: Kevin McCurdy as Huggy Bear Jonny Owen as Wideboy
Indeed, his definition of deity is decidedly eccentric.
It decidedly has not.