לזכות ר' אריה בן דבורה
There are certain halachos regarding Krias Shma that are only לכתחילה but not מעכב בדיעבד. For example, one should hear the words he utters but if he doesn't he is nevertheless yotzei [as long as he utters the words S.A. 62/3].
Also, he should stand still when he says the first pasuk but if he doesn't he is still yotzei [63/3 Mishna Brura 9].
In addition, he should clearly enunciate the words but if he doesn't he is yotzei [62/1].
Now it is important to note that בדיעבד means "ex post facto" - after the fact. One doesn't approach a mitzva to do it בדיעבד. He hasn't done it yet!! It can only be בדיעבד after he already did it. But let us say he is in a situation where he can only do the mitzva in a בדיעבד form - should he do the mitzva anyway? Should he do a mitzva initially in a בדיעבד way?
You would probably agree that doing it בדיעבד is better than not doing it at all. In fact, Tosfos [Gittin 28b] says that if one cannot perform סמיכה on a korban he should bring it anyway because סמיכה is not מעכב בדיעבד.
In the same vein, the Chazon Ish [Menachos 25/18] that one should bring the אימורים even though he will not be able to salt them because he is nevertheless yotzei בדיעבד.
However, Tosfos in Chagiga [4a] and Pesachim [89a] say that this is only true if he is SURE that he is obligated to bring the korban but if he is in doubt, he shouldn't bring it in a בדיעבד fashion.
So how would that impact Krias Shma? Maybe if one is not sure whether he is obligated to say Shma and can only say it in a בדיעבד way, then he shouldn't say it at all.
Or maybe there is a distinction. With regard to סמיכה we are talking about an act which is part and parcel of the הקרבת הקרבן. While saying Shma while standing or hearing the words is just a halacha regarding Shma but not inherent to the essential mitzva. דין בעצם vs. דין צדדי.
קריאת שמע על תנאי
What about saying Shma with a tnai [stipulation]: If my minyan makes it up to Shma before the last time to say it [סוף זמן], then I want to be yotzei then with the birchos Krias Shma [the ideal way]. But if we don't make it on time, if the Chazan sings a twenty minute א-ל אדון, then I want to be yotzei now.
The Maharsham [Daas Torah 46/9] says NOT to do it. If he does then it will be in the category of ברירה and we rule אין ברירה בדאורייתא. Uh oh.
Rebbe Akiva Eiger says that one MAY read Shma על תנאי [S.A. 46 על הגיליון] and proves this from the fact that one may make a condition for Sfiras Ha-omer.
[מאוצרות הג"ר אברהם גנחובסקי]