Hi, dylan,
15ac is a seven letter name of a French astronomer. The word “I’ll-bred” is extra. 6dn has a three letter abbreviation of the Latin version of Charles, surrounding the usual one-letter abbreviation of old, followed by an old name for the note usually called “ti”, followed by a two-letter abbreviation of shorthand, eight letters in all.
Yes, the letter in square 28 is F. 28dn has an extra word, “very”, which is a good hint as to the nationality of the person as it’s the last letter of the name.