Do the issurim (mosif or kollel) have to be actual, i.e. must the various relatives that constitute the mosif or kollel actually exist, or is it enough that is they did exist they would be assur (even if infact they do not exist) and therefore the bo'el has yet another issur added?
Your question is a valid one, and in fact is a subject of discussion amongst the Rishonim.
1) Our Sugya (14b) seeks a case of a Isur Mosif, and suggests that "if the grandfather would have a son," Eshes Av would be an Isur Mosif. The Gemara would seem to be telling us that we would have Isur Mosif only in such a case, suggesting that it is a real situation not a hypothetical one.
The RAMBAM (Hilchos Shegagos 4:3) rules clearly, " In all cases of Isur Mosif, the people who are newly included in the Isur have to actually exist, in order that we may say since it is now prohibited to these new people it becomes added to those whom it was previously prohibited to. If they did not exist, we do not say since hypothetically it could exist the new prohibition will be added to those on whom it was previously forbidden." The KESEF MISHNAH (ad. loc.) cites our Sugya as the source to this ruling of the RAMBAM.
2) The RAMBAM (Perush ha'Mishnayos as printed in Vilna Shas) learns that the son must be a minor, and two Isurim took legal effect on him simultaneously when he became Barmitzvah. (see Yevamos 32b). This is the only way for the Isur to become Mosif, he says. If this is the case our Gemara would no longer serve as conclusive proof that a normal case of Isur Mosif must have a practical application.
Furthermore, the TOSFOS YOMTOV and the Perush ha'Mishnayos (Kapach edition) both state that the Rambam rescinded this explanation and erased from his Gemara the words, "if the grandfather would have a son." This leaves us asking where the Rambam extracted this particular ruling in Hilchos Shegagos.
Do you have a specific place for background explanantion/discussion of the distinction between issur mosif and issur kollel?
Here is a copy of what we wrote on Isur Mosif and Isur Kolel (Background to the Daf, Kerisus 14b):
ISUR KOLEL, ISUR MOSIF
Even if an object that is already prohibited by one Isur Torah cannot become prohibited by a second one (Ein Isur Chal Al Isur), there are possible exceptions to this rule (Chulin 101a):
(a) Isur b'Vas Achas - Two Isurim that take effect at the same instant
(b) Isur Kollel - The second Isur includes objects that the first Isur did not (e.g. Yom Kippur prohibits not just the eating of Nevelos, but kosher foods as well).
(c) Isur Mosif - The second Isur adds a new dimension of Isur to the first (e.g. it prohibits the object to people that were not prohibited by the first Isur, or it makes the object Asur b'Hana'ah and not just Asur to be eaten).
(d) Isur Chamur (according to Rebbi Yehudah) - A more stringent Isur can take effect on top of a less stringent one. (e.g. the prohibition of Gid ha'Nasheh is more stringent than the prohibition of non-kosher animals, since it applied to Bnei Yakov when they were still permitted to eat non-kosher animals.