It is brought down that when reciting the bracha of “Borei Pri Ha’adama” on the karpas one should specifically have in mind that this bracha should exempt the maror that will be eaten later on. (See Mishna Berura siman 473:55)
Question: For those people who use Romaine lettuce for marror, being that it is something that is part of the meal and anyway would be exempted by the hamotzi?
Also, if the marror is eaten 2 or 3 hours after the karpas, due to the lengthy discussion of the Haggada, and the karpas has already been digested (it is past shiur ikul), how will the bracha on the karpas cover the lettuce? There are opinions that the bracha rishona ceases to be effective after shiur ikul has passed!
Your two points are the topic of discussion among the Rishonim. There are Rishonim that specifically say like you are saying (Tosefos), however others argue on this and hold that we need the hoadomo of the karpas to exempt the marror, even though it is eaten during the meal. This is for a number of reasons; it is not being eaten as a part of the meal, rather because of the mitzvah to eat it therefore it isn’t subservient to the “bread” of the meal. Besides that at this point we are also not allowed to eat the marror together with the matza.
Regarding the second point, the Rosh, asks this question and he answers that since the marror was in front of him when he said the bracha and he had in mind to eat from it when he said the bracha, so it isn’t considered a hefsek even though he said the hagadah in the middle.
Tosefos Pesachim 115a D:H Vhadar, Rosh Pesachim 10-26, Rasbam ibid 114 D:H Pshita, Bach O:CH 473-10, Kaf hachayim ibid 109. Shut Rashba 1-72, Aruch Hashuchan ibid 18.