Relative Clauses

Well, actually, there's no need for magic.
You could say 'quem' as much as 'who', it wouldn't be wrong. There's just a minor (big) problem: it can easily sound far from classy.

But I'll tell you some tips.
It's less likely to be used:
1. w/ Plural nouns;
2. w/ Formal/cult vocabulary;
3. w/ Future subjunctive;
4. w/ Collective nouns.

"A menina quem te abraçou aquele dia (es)tá uma moça já." (OK)
"O/um trabalhador quem vive em regime de semi-escravidão é miserável demais" (A bit strange)
"Os/uns trabalhadores quem vivem em regime de semi-escravidão são miseráveis demais." (Strange)
"As/umas legiões de trabalhadores quem vivem em regime de semi-escravidão são miseráveis demais." (VERY strange)

Some constructions, however, like the following using the verb 'ser', may feature 'quem' even more oftenly than 'que'.
There, you can agree 'quem' with the subject or with the 3rd-person singular. Accordingly, it may have small semantic differences.

"(Fui/foi) eu quem abraçou-a, na verdade." (OK)
"(Fui/foi) eu quem abracei-a, na verdade." (OK)
"(Fui/foi) eu que abracei-a, na verdade." (OK)
"(Foste/foi) tu quem assistiu, lembras?" (OK)
"(Foste/foi) tu quem assististe, lembras?" (OK)
"(Foste/foi) tu que assististe, lembras?" (OK)

Changing the order, 'quem' certainly would be seen as a demonstrative pronoun, so everything changes.
"Quem abraçou-a fui/foi eu." (OK)
"Quem abracei-a fui/foi eu." (Wrong)
"Que abraçou-a fui/foi eu." (VERY strange for this context)

N.B. I used grammatically correct sentences for didatic purposes.

And you can use the other words you've listed mostly just like their English counterparts.
I expect this could help. =]