I think it isn’t usually used with body parts (as galaxyrocker wrote, eg. mo chosa and not *mo chuid cos¹ – this example is also given in Wiktionary and GnaG), or with family members, so mo thuismitheoirí rather than *mo chuid tuismitheoirí, mo dheartháireacha, rather than *mo chuid deartháireacha, m’uncailí, mo dheirfiúracha, m’aintíní, etc.

Does your dialect use cuid in those instances? If so, do you happen to say those without cuid, or do you always use cuid?

¹ galaxyrocker and Wiktionary give mo chuid cosa, but the gen.pl. is cos, so I’d expect, if one uses cuid for legs/feet, mo chuid cos ‘my share of legs’ instead.

The genitive plural is all but dead, and the singular isn't too far behind it. I've even heard 'mo chuid airgead' multiple times from a teacher (he taught in a Gaelscoil and at NUIG for the C1, born and raised in Carraroe), ar dheis Dé go raibh a anam. So cosa is definitely used there (also what I got told from another native on my post about cuid on Reddit). Likewise, tuismitheoirí is rarely used (muintir being more common).

Also, interesting Bríd! I would've assumed it was a younger thing, like the increasing use of 'dur'. What about the pronunciation of ó as uaidh?

Interesting, An Cionnfhaolach. Could that possibly explain how a surname like Mac Domhnaill (MacDhòmhnaill in Scotland) ended up with a "d" at the end of its anglicized version, McDonald?

I wouldn't think so. The -ld there looks like an English-language phenomenon.

Does terminating -ll become -ld anywhere else in Scottish Gaelic?

The only thing analogous I can think of is found in In the Irish of east Cork, where in some sub-dialects, -ll especially in Sandhi position, become -ld; e.g. mo bhuachaild, go hEochaild.

Yes, we don't use the genitive as much here. I'd see nothing wrong with saying "mo chuid airgead" too.
I heard we also use the dative a lot in Conamara.
Really going to school and learning grammar rules has ruined my native Irish, now I question myself if what I'm saying is right or wrong, I get it mixed up in my head. I don't know if what I'm saying is native or learnt later.

Uaidh -
Did you mean something like? - Theastaigh sé uaidh.
Chuaigh sé ó theach go teach.
Give me an example sentence of when "uaidh" is wrong.

Technically "uaidh" would be wrong in the second sentence.

However, from what I've read and heard, it's still often used. Basically, "uaidh" has replaced "ó" as the base form of the preposition.

In modern Irish, another preposition has been replaced by the 3rd sg. masculine form:
faoi is used as a preposition but historically it's only the 3sg m prepositional pronoun (ie. under him), the original preposition is fo (retained in Scottish Gaelic).

In Manx, even more prepositions show that evolution.

There are quite a few of those all around the bigger Gaeltacht, aren’t there? air and aig in Scottish (with ar pronounced like air in Irish too, not sure about ag),

Technically "uaidh" would be wrong in the second sentence. However, from what I've read and heard, it's still often used. Basically, "uaidh" has replaced "ó" as the base form of the preposition.

But Connacht is problematic. Here ag + a vowel is lenited, becomes agh + V. Then this /gh/ [ɣ] is reanalyzed as /dh/, because lenited /d/ is pronounced the same. Ag mo, ag do, etc. start to be pronounced similarly to Scottish as /gə mo/, /gə do/… Also do ‘to, for’ in Connacht becomes pronounced as /gə/ (as if spelt go). Connacht grammarians (or so I think I’ve read somewhere, but now I cannot find any actual source on it) of the 19th century decide that it’s not really ag + pronoun, but do (=‘to, for’) + pronoun. The confusion might have been reinforced by the fact, that do actually often governs the verbal noun (but not in this very construction).

On the other hand, as noted at the end, in the Middle Irish (and perhaps Classical Gaelic?) ag and do were commonly used interchangeably in all constructions involving verbal nouns (and ag finally won everywhere in continuous progressive sentences), so some of these Connacht contractions might actually come from do (if they contracted before ag won over do here), so there might have been no confusion here actually…