rogard, below are my comments.
1. You should never put your name on a subtitle you didn't make.
Or improve by editing. I agree.
2. If you change something in an existing subtitle, leave the original credits untouched and add your name, clearly stating what you did exactly. (spell-checked/synced/formatted/completed/HI removed etc)
I agree.
3. Don't annoy the hell out of the audience. Your name and any kind of advertising should never be on screen for more than a few seconds, and not more often than one or two times (beginning and end).
I agree with this in general. I personally like credits only at the end.
a. Is it okay to upload subtitles from another site?
Yes, I think it's ok. Especially if there are no other good versions posted here. However, you should first check to make sure that that version is not already posted by someone else. OS will not let you post the exact same sub, but it only takes one character different for OS to let it be uploaded. What annoys me is when some people take my subs that I have painstakingly created and remove the credits and re-post on another site. Then someone else finds it and thinks it's a good sub and re-posts it here. It's still the same sub, but with the credits removed so OS lets it be uploaded. Sometimes they will add their own credits to it. That is REALLY annoying.
b. If you do that, how should you go about the credits? Should you say "uploaded by XXXX on [subtitle-site]"?
I'm not sure what to do here. Your suggestion has some merit.
c. Do you think it's okay to edit existing subtitles in general? Is there something similar to a "copyright" or "ownership" for subtitles?
I think it's ok to edit other subs, especially to correct mistakes or to re-time to another release. If you do, you should give credit to the original creator, if known. Most subs posted here are not created from scratch by the user (they are taken from a source like a subpack or DVD). Some are original (like an original translation or transcribed from listening). If they are, I think they do kind of "own" the sub and should have some rights to what's done with them. (Like having their credits in the sub and in the comment section of the upload.) But by posting them here, they may be giving up their "copyright".
d. Should the subber mention if their work is based on somebody else's work, even if they only took the timing or text from an existing subtitle to use it as a base for a translation?
They should mention if their work is based on someone else's work, but not if it's just timing or for translation.
d. Should machine translations be deleted as soon as they are spotted or can anybody think of a useful application?
The only use I can see for them is as a base for correction by someone else (timing or getting the general meaning of what's being said). I know it annoys a lot of people based on the number of comments posted by users. There were some users here who posted subs like those and put big warning comments not to download unless you were a submaker. Most users ignored those warnings and downloaded them anyway and then were angry and posted nasty comments to the uploader. I guess, after thinking about it, I would say to delete them right away. If submakers wanted a base for working from, they could make their own based on other language subs that are already posted. However, what some users call machine translations are really a manual translation or transcription by someone who does not have a full grasp of the language he is translating to or transcribing. It would be a shame to delete that person's hard work.
e. Aren't uploader highscore lists the wrong way to encourage good quality subtitles? Shouldn't there be a quality index instead of the # of uploads?
The subtitle rating system is supposed to be the gauge that users use to decide whether to download or not. OS recently eliminated anonymous ratings, which I TOTALLY agree with, so not as many subs are rated anymore. The sheer number of uploadings should not be a basis for deciding whether to upload that user's subs. Some users just find garbage from other sites and post them here in mass without checking the quality. I have seen some comments by users that say something like "this is a good sub, I can see why the uploader is a 'Gold Member'". 'Gold Member' has nothing to do with the quality of uploaded subs, just the number of uploaded subs.
f. Is there something like a universal code of conduct for the subtitle community? I couldn't find anything on the web.
I have never seen anything either. Maybe what you are starting will be a basis for that. Maybe if the operators of this site like what is being created here, they will post it somewhere here onsite as guidelines.
g. Is anybody here making subtitles for a living? I am just curious...
I doubt it.
I can't think of anything else to add to your list of comments and questions. Good start on a set of guidelines.
LeapinLar
There ain't no perfect subtitle ...although LeapinLar's are always damn close to perfection. I agree, LeapinLar is the man. I am male btw. 
don't be like that 
yes yes masters I will make it clear by putting it in the notes