bal tosif for sukkah on shmini atzeres

The gemara (Eiruvin 96a) proves that bal tosif does not apply when doing a mitzvah outside its normal time unless one has kavanah l'shem mitzvah.  Were that not true, says the gemara, one who sits in sukkah on Shmini Atzeres should get malkos for bal tosif.  Rashi comments:

 ועוד הישן בשמיני בסוכה ילקה. דמוסיף שמיני על השביעי ואנן מיתב יתבינן בשמיני בספק שביעי לכתחלה אלא שלא בזמנו בלא כוונה לאו תוספת הוא ולהכי שרינן דאי שמיני הוא לא מכוינן למצות סוכה

The gemara in speaking about malkos for sitting in sukkah on Shmini Atzeres in Eretz Yisrael, but Rashi adds in a justification for our doing so in chutz la'aretz not being a violation of bal tosif: Rashi writes that we have in mind that we do not kavanah for mitzvas sukkah if the sfeika d'yoma day is not Sukkos.  

In one of the earliest posts on this blog we discussed (see here, here, and here) Tos question of why there is no bal tosif in blowing additional tekiyos beyond what the Torah requires, i.e. blowing both before and during musaf on R"H.  Tos (R"H 16 answers) that there is no issur of bal tosif in doing a mitzvah a multiple times.  For example, the gemara says that a kohen can duchen multiple times a day  because each tzibur that he duchens in is a new kiyum mitzvah.  

Rashba gives what seems to be a far simpler answer to Tos question: there is never bal tosif in fulfilling a din derabbanan because the Torah gives Chazal license to legislate and add to/change mitzvos.  We blow tekiyos d'meyushav and dme'umad because we are following the takanas Chazal to do the mitzvh that way.  (The Rashba's answer seems so obviously correct that the Achronim struggle to explain why Tos avoided it.)

According to the Rashba, Rashi's question of why there is no bal tosif when we sit in sukkah on Shmini Atzeres does not get off the ground (see GR"A to O.C. 666).  Since the Chachamim made a takanah for us to keep Y"T sheni and sit in sukkah because of sfeika d'yoma, by definition there cannot be bal tosif in listening to the Chachamim and doing so.

See Netziv (on the Sheiltos 45:6) who has a novel approach to sfeika d'yoma by Shmimi Atzeres (note that the gemara always talks about why there is no malkos for bal tosif if you sleep in the sukkah on Shmini Atzeres, but never talks about bal tosif for any other Y"T sheni practice -- why is Shmini Atzeres singled out?) and works out a way to harmonize Rashi and the Rashba, but b'pashtus Rashi belongs to the camp of Tos in R"H that for some reason that does not accept the Rashba's chiddush.