To answer your first question you must realize that few Rishonim agree to this Psak that even according to R’ Akiva, Miderabanan one must eat by Chaztos. Although this was definitely by the Korban Pesach(Rambam 8-16 and Zevachim 56), since the Afikomen is only a Zecher therefore we may be lenient. This is Meduyak in many Rishonim (Rosh,Tosphos and the Ran) See Igros Moshe(Ohr Hachaim Chelek 5 Siman 38 Sief Katan 8)
Regarding your second question that Tnai is that something should HAPPEN and then this chalos will be chal but here the Tenai is if the HALACHA is this way then my KAVANA is to be Yotzay now. Similarly by Krias Shema some Tenaim are able to be made.
Chag Kasher Vesameach!