"Nice Dress Nellie" is an actress that appeared in at least 34 episodes of the show. (Mayberry Wiki article) "According to assistant director Bruce Bilson, her name is Marvel, but he did not give her full name. Marvel was a stand in for Don Knotts and Mr. Bilson picked her for the stand in job." (Mayberry Historical Society article)
I think her name is probably Marvel Lawrence.
I saw a comment on a MeTV article about Nice Dress Nellie where a poster named Jermyn said "I searched IMDB for someone named "Marvel" (first or last) and the only possible one I came up with was a "Marvel Lawrence" who was an actress with one credit for appearing as a store clerk in The Danny Thomas Show (a.k.a. Make Room for Daddy) in 1961 in Season 9, Episode 3, Love Letters."
Given the connections between Andy Griffith and the Danny Thomas Show, I decided to do some research. I then found this Dec. 31, 1965 Article (you have to download the pdf) in the Alpine Sun (Alpine, CA) about Floyd where he said "Marvel Lawrence, who stands in for me, is the foster daugher of Eltinge." (page 14) He is talking about Julian Eltinge. Also note how he said that Marvel was his stand-in like how Bilson did in my first paragraph.
At these two links (1) (2), people noted that they had seen Nice Dress Nellie on The Bob Newhart Show. I then found this June 30, 1974 Article in the San Bernardino County Sun that said "The Bob Newhart Show" wrapped for the season on Friday with a mood of sadness permeating the set." "The husband of Marvel Lawrence (stand-in for co-star Marcia Wallace) died on Tuesday". You have to click "Show page 47 article text" to see it.
Now we need to find out who Mr. Schwamp and the pianist named Hazel are.