Is the comma in the German here strictly required, or more of a stylistic thing?
It's required and omitting it would be a mistake. Unlike English, German has relatively strict comma rules.
There is always a comma before "dass", unless it stands at the beginning of a sentence.
Yes, I am absolutely sure :). If you're interested, see the online Duden's introduction to the spelling reform, rule no. 121:
Shouldn't it be "ich will, dass due mein Freund waerest", since it conditional and not happening in the present? The English translation of the sentence above would be "I want that you are my friend".
The conditional would have been: "Ich möchte, dass du mein Freund seiest/wärest."
this would be "i would like it if you were my friend". the meanings are subtly different
One good rule of thumb with regards to commas: if a phrase/chunk of a sentence has a conjugated verb, it gets separated from the phrases with conjugated verbs. Like "[Noun/Pronoun] +[conjugated verb] [comma] [noun/pronoun] + [conjugated verb]". However, if the verb is in its infinitive form: no comma needed.
except: "Ich spare, um ein Auto zu kaufen". Constructions with (um) zu and infinitive require a comma too.
I don't really understand the difference between the different forms of saying 'that' in german: dass, das, da, etc. Is there some sort of rule for which one you use? Like always use 'Das' at the beginning and as the gender, always use 'dass' in the middle of a sentence?
You wouldn't believe how many natives don't know that either. Does this help? http://yourdailygerman.wordpress.com/tag/das-and-dass/
You're welcome. Most important is to know that 'dass' starts a new, subordinate clause and usually is the first, sometimes the second word of this sub-clause. So at first, you have to recognize where such sub-clauses are when translating from E. to G. The 'das'es are much fun sometimes: Ich glaube, dass das das Kind beruhigen wird.