More commonly known as 機能性飲料, these kinds of "fad" drinks are sold to convey a kind of health benefit, commonly advertised as increasing organ (heart, pancreas, eyes, ...) function. Think herbal or vitamin drinks.
握草，in that context as you suggested, is a curse word like "f*ck", "damn". There are couple of more variants of it such as 我草，我靠，我操.
握草 can be a way to get around the censorship of rude words in web posts.
The differences are contextual. I do agree it is very subtle.
到 is used when the verb, (action), produces a response from a purely physical perspective. Whereas, 了 is from a non-physical moral, legal, ethical standpoint.
So, in 收不到 vs 收不了, the former connotes the physical impossibility of receiving whatever is sent, perhaps it was physically destroyed ...
When you suppose that you would be dead in a battle anyway, what/how would you perform in it? You would certainly be desperate, try your best to kill as much as you can and destroy as much as possible. Metophorically，it suggests that you will do something as hard as you could. Basically，you'd be desperate to achieve what you intend to do. That is the kind ...
必死 = certain death
决心 = determination
必死的决心 means "A determination so strong that you prepare to meet certain death to carry it out"
It is a 誇張詞 (exaggeration phrase).
Similar to "誓死" (swearing on one's own life)
You can omit "的" and say "必死决心"