Perhaps? hehh... I'd say no, but that's just my poorly educated guess. Dothraki are less preposition-happy than English, so I try to do without when is seems feasible. I probably try too hard, though. We're not on a strong footing with these kind of adverb things. Simple things like rekke, silokh or ajjin certainly work as is, as they do in English (even though exopressions like "today" seem to have an incorporated preposition).
If you mean that the phrases mean a bit different things, I guess they do. Even in English you can (I think) say either "Few days from now..." or "In a few days..." and the former has a sense of "after few days have passed" and latter "within a few days time", but since the time scale is imprecise and I would use neither of potentially imminent occurence, I find them practically identical in meaning.
I would think it's hard to be nit-pickier than me, and why not, if you could; when most of us are nowhere near linguists, discussion is the best way to widen one's perpective and see the problems and possibilities.
I don't know. As a linguist I don't think you can just say "few days from now" in the sentence and leave it like that, not in any language. I don't think Dothraki is "less preposition-happy than English" it just uses prepositions where it needs to, in this case it needs to, unless you could use an allative, but certainly it needs either a prep. or a case. But then again, this is my hunch.
About kiai:
I think it's quite a noun, I found: 気合(P); 気合い 【きあい】 (n) scream; yell; fighting spirit;
from what I could find it is well attested, common onomatopoeic expressions tend to be written solely in hiragana, this one has its own kanji. The kanji by themselves seem to mean "agreement of spirits" or "joining of spirits".
It of course could be the case that the noun just so happens to be great to represent the action, which is quite cool.