Form the list of Registered Features, you could choose:
- Stylistic Alternates (salt): "[...] alternate glyph designs for a purely aesthetic effect.", or
- Stylistic Sets (ssxx): "In addition to [...] stylistic alternatives of individual glyphs (salt), some fonts may contain sets of stylistic variant glyphs corresponding to portions of the character set, e.g. multiple variants for lowercase letters in a Latin font. Glyphs in stylistic sets may be designed to harmonise [sic] visually, interract [sic] in particular ways, or otherwise work together."
So, if the 'a' is the only variant, I'd go with 'salt'.
As for the other, specific question, Candara has double-storey for roman, and single for italics. This is a common choice since the italics is suppose to be based on a "stylized form of calligraphic handwriting".
APPENDED: Candara has also an Oblique version, as the OP remarks. I understand the original quest is trying to get a double-storey 'a' in the italics, and the OP also suggested substituting the oblique 'a' in the italic text. Unfortunately, italics and obliques (at least in this font family) don't have the same slanting angle, and the substitution may look weird. Unless the italic face of the Font Family has a specific double-storey stylistic alternate, I guess there is no way to add a double-storey italic in this typeface ( without much trickery :P ).
EDIT: related to the article you mention from FontFont -- for example, FF Unit lists in this page that the 'a' stylistic conversion is in the salt and ss01 features. Other FF fonts also handle it similarly.