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
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
it is decidedly so.
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.
A decidedly republican form of government.
It decidedly has not.