The Gemoro says that a Tenai is only valid where the action could be done through a Shaliach. How does this relate to tenaim that some people makewhen doing Mitzvos, e.g
1. If saying Shema before davenning in case he will miss the Sof Zman (but without Tefillin and Birkas kerias Shema, hence prefers to be Yotze later if possible
2. Eating Afikoman after Chatzos (Eat just before Chatzos - if Halocho like R.EB"A, Yotze and can eat more after Chatzos etc
In such cases, one cannot be Yotze the mitzva through a shaliach!
The rule of "Leisa b'Shelichus Leisa b'Tenai" is a Halachah that applies to the Tena'im that we learn from Bnei Gad u'Bnei Reuven. When applying the Chidush of the concept of Tenai that is learned from Bnei Gad u'Bnei Reuven, one must use the formula found in the Torah with regard to Bnei Gad u'Bnei Reuven, which also includes the laws of Tenai Kaful (expressing both positive and negative sides of the Tenai), Hen Kodem l'Lav (expressing the positive side first), Tenai Kodem l'Ma'aseh (expressing the condition before the act), etc.
Generally, these are only necessary in bilateral deals. The Tena'im you mentioned, along with many similar ones are actually merely a limitation of one's Da'as and do not require the Halachos of Tenai'im.
Dov Zupnik
Note: Similar Tena'im to the ones you mentioned can be found in OC 233 (see Bi'ur Halachah) regarding Tefilas Minchah during Bein ha'Shemashos, OC 489:3 (see Magen Avraham) regarding Sefiras ha'Omer and 590:6 (regarding Teki'as Shofar). In the latter case, the Mizrachi cited by the Beis Yosef does not accept such a Tenai, perhaps for the reason you mentioned. (The case of Keri'as Shema that you discussed is mentioned in OC 47, Afikomen is the Teshuvos Avnei Nezer's suggestion.)
Many Acharonim discuss these "Tena'im of Kavanah," and the issue that you raised, concluding that the rules of Tena'im may not apply to them as Rav Zupnick wrote; see Oneg Yom Tov OC 1-3, etc.