You'd need the definite article - Dw i'n gwneud y brecwast yma - although that's also slightly ambiguous, i.e. it could mean "I'm making this [particular] breakfast" or "I'm making the breakfast here [not you]". If you needed to, you could avoid the ambiguity by using "hwn", "Dw i'n gwneud y brecwast hwn".
Not really. "'ma" is just a contraction of "yma", "'na" of "yna".
yma / yna = here / there
y ... yma / yna = the ... here / there = this / that ...
e.g. y bara yma = this bread; y dyn yna = that man
You can contract "yma/yna" in informal language, especially after a vowel e.g. y bara 'ma, y dyn 'na.
You're right, it can be analysed as [i], but it also can be analysed as [j]. John Wells the phonetician, and Welsh speaker, mentions this in his work. I personally think it's a better way to represent the behaviour of Welsh diphthongs, especially when it comes to representation of things like complementary quantity, e.g. llai, llaid S: [ɬajˑ, ɬajˑd], N: [ɬajˑ, ɬajdˑ]; lleia, lleidiog S:[ɬejˑa, ɬejˑdjɔg], N:[ɬejˑa, ɬejdˑjɔg]. Transcription such as [ɬaiˑ, ɬaiˑd, ɬeiˑdjɔg] etc. wouldn't really work.