In reference to the questions about Rav Chaim's explaination, possibly one can distinguish between other mitvos and tefillah. My Rebbi HaGoan Rav Naftoli Friedler ZT"L often refered to the view of HaGoan Rav Shlomo Heimen ZT"L that the machlochas whether mitvas ztrickah kavunah is whether it is sufficient to have the maisah ha'mitzva or if one additionally needs kavunah to make the maisah into a maisah mitzvah. (see Rav Friedler's sefer IMREI SHEFA helik alef, Cholin simin bais, binyan kavanah bshkitah ). Thus l'koli alma a maisah hamitzva is required. With tefillah there is no maisah other than the kavuna ha'tiffilah (the moving of ones mouth and standing are only hekshirai mitzvah). Therefore even if in other mitzvas the P'sak is that one does not need kavanah that is because there is still a maisah. But by tefillah since without kavanah there is not even a maisah, one at least needs the kavanah that he is standing before H"S to have a maisah. Therefore even though d"Oraisa one does not need to say all the brochos of S"E, it is necessay to have some level of kavanah to be yotzi S"E d"Rabanan even to those who hold mitzvas ain tzirikah kavanah since here they would agree that without kavanah in tefillah there is no maisah. This level of kavanah is less than that needed by those who hold mitvas tzrikah kavanah.
It is possible to ask on this that even if one is yotzi a mitzva such as lulav without kavanah, one should not be yotzi the brocha on the mitzvah of lulav if it was said without kavanah since the brocha has no more maisah than tefillah. However since the brocha is said on a mitzvah which has a maisah (e.g the waving of the lulav) , it is possible to answer that that maisah is sufficient to give the brocha validity, whereas with tefillah since there is no maisah without the kavanah there is nothing to give the brochah validity other than the kavanah.
Your suggestion is very good, that everyone agrees that even a Mitzvah d'Rabanan needs Kavanah when there is no action involved in the performance of the Mitzvah other than the Kavanah itself, such as the Mitzvah of Davening. Rav Yehudah Landy mentioned that there are sources that explain that since Tefilah is called "Avodah sh'b'*Lev*" (Taanis 2a), everyone agrees that Kavanah is necessary, for without Kavanah, there is no "Lev."