Since written Cantonese is considered informal(?), it reflects the spoken form directly. Some rare characters written Cantonese has are usually replaced by chars with a similar sound, but that's another thing.

Technically, both characters are classified under the rice radical. I wonder if 粤 was originally a variant of 粵 which became the standard for simplified Chinese. This would explain why it also appears in hanja which are normally traditional characters.

Currently I'm focusing on Spanish and Mandarin and I'm learning rapidly but Spanish verb conjugation is really kicking my ass. Any advice?

Um, call everyone Usted? That way you can trim down by a third the number of forms you need to memorise.

What interests me about Cantonese is that 99% of the written language I've seen is in un-simplified, traditional characters. Does this mean that simplification is not totally mandatory and that some language communities on the Chinese mainland can opt out of it? Incidently, I saw one website that discouraged learning Cantonese. I don't remember the URL.

That may be because, in my experience, only Hong Kongers and Macaners write in Cantonese. Written Cantonese has no official status elsewhere and is deprecated in favour of Standard Chinese. in Guangzhou, even the use of spoken Cantonese is on the decline. There have been complaints there about children being punished for using it during classtime.

That depends. Will I sound excessively formal? So far I've totally avoided these forms because the emphasis is on speaking.
Well Hong Kong isn't part of the mainland really - due to the "one country, two systems" policy, they're allowed to retain many different features of their political and economic life. As for the PRC proper I don't think there is any law against using traditional characters for private use.

Better to sound overly formal than rude, right? The only problem I see with skipping learning the tú
(and vosotros) forms is not understanding what someone else is saying, although it shouldn't be too hard to infer. But, personally I think the tú is very easy, in all tenses except preterite the tú form is the "usted" form with an -s. Estaba > Estabas. Pudiera > Pudieras. Come > Comes. And in the preterite one adds -iste to er/ir verbs and -aste to ar verbs; no matter if it's irregular. Ver > Viste. Poder > Pusiste. Which simply build off the yo forms of vi and puse, respectively.

Don't try to memorize verb paradigms & charts. Instead, just learn natural sentences that use each form of the verbs.

Hmm? I thought that after learning Arabic's conjugations you would be already mentally prepared for Spanish's.

Try making many sentences coming up with the conjugations as fast as you can; the idea is to "internalize" them in your head.

Among the "regular irregularities", there's these coming from those Western Vulgar Latin -> Spanish sound changes (stressed /o/ -> /we/ poder-puede; stressed /e/ -> /je/ venir-viene, /i/ pedir-pide).

There're also those "orthographic irregularities" based on the underlying phonemes:

lanzar (to throw) /lan.'θar/ - lancé (I threw) /lan.'θe/

And that's because the syllables are analyzed as:
/θa/, /θe/, /θi/, /θo/, /θu/, /-θ>
<za>, <ce>, <ci>, <zo>, <zu>, <-z>

pez (fish, singular) /peθ/ - peces (fish, plural) /'pe.θes/