You wrote:
"... The RA'AVAD (beginning of ch. 2) writes that [the reason two dishes are required according to Beis Shamai] is in order that it be "recognizable" that the food was set aside for Shabbos."
Perhaps this is also pshat in the gemarah that discusses the possibility of a maximum shiur of eruv tavshilin, we would have thought that too much does not indicate preparation for Shabbos !
Excellent point. The Gemara says that there is no maximum Shi'ur for an Eruv Tavshilin. This is problematic, because why would we have thought that there is a maximum Shi'ur? The answer is, as you wrote, that we might have thought that if one prepares a full day's worth of meals for Shabbos, then the Eruv Tavshilin no longer serves as a "Heker" to permit cooking extra foods on Yom Tov for Shabbos. This answer was also suggested to us recently by Rav Mordechai Rabin (Jerusalem).
Other answers to this question are offered by the Acharonim (see TZELACH and PNEI YEHOSHUA, TOLDOS YAKOV, as well as CHIDUSHIM U'VI'URIM -- but these answers to not seem to conform to Rashi's explanation of the Sugya).