Hello Eilidh - just a couple of comments. At the start where you use "an dàrna pàirt de m'obair" (dont for get the stracan as the a's in dàrna and pàirt are long), my own instinct there would be to say "Seo an dàrna pàirt de mo chuid obrach", the inplication being that this is portion of work, which is certainly how I have heard this expressed.
Next, "Chan fhaic mi idir iad mar a b'àbhaist" - you need to line up the tenses here - chan fhaic, future dependent of the verb faic, so in English, I will not see, but "mar a b'àbhaist", is a past tense, as this is constructed (if we ignore the mar), a = relative pronoun, bu - past or conditional tense of is, the copula and "àbhaist", which is a noun meaning habit or custom. So if you are saying that that you will not see them as usual, then you need to say "mar as àbhaist" - that's my take, unless you are trying to express something else. If you are saying "I did not see them as usual", then that would be "Chan fhaca mise idir iad mar a b'àbhaist".
Sealbh = possess - sealg = hunt? creich - from creach - plunder
Nach fhaod tu fhèin breith oirrn - if what you mean is "Will you fetch us", then best to express this as "An toir thu oirnn" - Nach fhaod would more mean to me "Won't you permit", and breith is the present particple of "beir", the future dependent of which is "toir", as I have indicated. Certainly, my take on it.
Cuin a ruigidh am... - Cuin a ruigeas am...? This "a" is a relative, so you need the relative future here.
"am mara" - the sea is "a' mhuir", is feminine with its genitive genitive, "na mara". Also "ma thèid a' mhuir nas gairbhe..." My own preference would be to use "fàs", "grow", so "ma dh'fhàsas a' mhuir nas gairbhe"
"obair taighe", if you are meaning school homework the general term is "obair-dhachaidh", from dachaidh = home, with lenition in dachaidh, as it is a closely bound qualifying noun in the genitive following a feminine noun. (I might be wrong about this and am willing to be corrected).
I hope this is useful...and any errors in this are mine!