No, this isn't what it means. There is no implication of continuity. A correct definition:
f(x)→a as
Unparseable latex formula:\x \to \infty
if
∀ϵ>0,∃M∈R such that
x>M⟹∣f(x)−a∣<ϵ.
This is basically correct, but it's very unclearly worded.
Not sure what you're trying to say here. Some of this isn't true, and what is true needs justification, which you haven't provided. I think the justification is supposed to be what's following, but you either need to put the justification
first or rewrite the above to be more like "We still need to deal with the interval [0,M]. We shall show that we can find c > 0 such that ..."
If you're going to use a theorem, you should actually name it, and make it clear that any conditions are satisfied. e.g. "Since f is cts, and [0, M] is a closed interval, f attains its bounds on this interval. In particular, we can find z in [0, M] such that..."
There is no reason to assume c <= 1/2. E.g. f(x) = 1 everywhere (f is constant). You also seem to have switched from inf to sup but I assume that's a typo.