A person can only say a blessing that is on behalf of someone else but does not pertain to himself if it is a commandment for the other person and that person is unable to say it for himself. If the other person is not Jewish (e.g. Adam's partner), then it is not a mitzvah (commandment) to say hamotzi, so hamotzi may not be said by Adam if he is not eating any bread himself.
hi shocheradam how r u? i am insulin dependent and take shots everyday. i agree with all those who already said - only you know your body and what works for you. i stay away from carbs in the week which cuts down on how much insulin i need, but i make shabbos with a bit more insulin than normal. i always find a good thing in these things is to be normal also! no one needs someone to be ULTRA strict in their diet only to burn out and two weeks later be eating everything they shouldn't hatzlacha raba on your journey to finding what works best fo ryou
a point to note on saying brochas for others. Birchas Hamitzvos you can do this, IF you are also required to do that mitzvah at that moment. for example blowing shofar, one guy says brocha and everyone is included if they have kavonnah. but if he had already fulfiled his requirement to hear shofar, then his brocha would not be necessary and thus wouldn't even include him, let alone others!
birchas hanehenin may not be said for another person unless it is to fulfil a mitzvah- ie kiddush. but if you eat and apple I cannot say the brocha for you. unless of course I am about to eat an apple and we both have the right kavonnah but it gets tricky here already... the custom is everyone says their own brochas
bentching again someone can only cover you if they themselves are required to say bentching at that point and you both have the correct kavonnah.
If you can't eat bread - then you can't eat bread. You don't need to and you don't need to bentch. a lot of my diabetic friends eat a kzais, probably half of one normal slice - this then requires them to bentch and they get the mitzvah of lechem mishnah