#1
Both WoWI and Curse offer SVN and Git repositories for addon authors, and both can package a ZIP file and push it to your public download page from your repo. (Note that neither site has "a repository" -- every addon gets its own repository; you may be thinking of the original WowAce repo, which *was* shared by all addons.) Both sites also allow you to just manually upload a ZIP file if you would rather do that.
You can also use the repository services of any other site. Many authors use Google Code for SVN repos, or GitHub for Git repos. In this case, you will have to manually create a ZIP file and upload it to the addon site(s) you're using.
#1a
CurseForge and WowAce are both part of Curse. WowAce used to be its own site, but it was absorbed by Curse years ago. It's still semi-separate for legacy branding/community reasons, but it's all the same thing on the backend. The only difference is that if you choose the WowAce domain for your addon, you can't manually upload ZIP files; if you want to do that, you have to choose the CurseForge domain. The forums are also unified on the backend, but each domain shows only some of the (sub)forums.
#2
WoWI has more stable servers, and a way more awesome staff who actually respond to requests for help, know what's going on, and can fix problems.
Curse has more features, but their servers go down a lot more, their packager breaks very often, bugs that take years to get fixed, and they tend to "accidentally" things a lot.
Curse's localization app lets people enter translations for your addon on the website, and the packager can pull the latest translations from the website into your addon when it makes a ZIP file for you. However, you can use the localization app without using the packager and without using a Curse-provided SVN/Git repo. My addons' SVN repos are all on WoWI, but I do use the Curse localization app. To update the localizations in my addon, I export the translations from the Curse app and copy/paste them into my addon files.
Curse lets you create as many extra pages for your addon as you want, and can automatically populate those pages with documentation generated from your files (several syntaxes are supported). This feature is useful if your addon has a lot of complicated options (WeakAuras, kgPanels) or if your addon has an API you intend for other addons to use, but most addons do not need more than the description and basic instructions written on their main download page.
Curse also lets you create a dedicated forum for your addon, but I don't use this feature, as it just adds more places you need to keep track of to provide support for your addon, and more places for users to look to get support.
#4
I would recommend *not* reading the manual for either SVN or Git. Unless you are a diehard commandline user, you do not need to know 99% of what is in the manual. If you're going with SVN, just get TortoiseSVN; it's a Windows shell extension, so you don't need to learn a new GUI, you just look at your files in Windows Explorer and do everything via right-click. If you're going with Git, there is a TortoiseGit. I don't use it, since I rarely have to interact with Git repos, but I imagine it's just as straightforward as TortoiseSVN.
#4
Most likely your addon does not actually need "documentation". You just write up a good description and basic instructions on your download page, and that's it.
If you're using WoWI, there is no "packager configuration". When you want to make a release, you go to your repo list on the website, click "Make ZIP", fill in the blanks, and click a button.
If you're using Curse, there is still very little packager configuration to be done, it's all one-time stuff, and there is
extensive documentation about it. Most of the questions about the packager I see on Curse are posted by people who obviously did not RTFM and/or are afraid the universe will implode if they just try something and see what happens.
Really, the answer to this question is: stop overthinking it, and just do it. It does exactly what it says on the tin, and you don't actually need to use most of the available features and options to get your addon out there.
#5
There's no magical "process" for getting your addon onto multiple sites. The process for every site is slightly different, but basically the same: you upload your addon. If you're using one site's SVN or Git repo and packaging directly from your repo to your download page on that site, then you get to download the resulting ZIP file and upload it to the other site(s) yourself. If you're not using either site's repo feature, then you just upload your ZIP to both/all sites yourself.
My "process" is this:
1. Click the button on WoWI to create a ZIP file and put it on my WoWI download page.
2. Download the ZIP file from my WoWI download page to my computer.
3. Upload the ZIP file from my computer to my CurseForge project.
That's it.
Edit
In response to Dridzt's mention of the author reward program on Curse, that's really irrelevant to this dicussion, because you don't need to use Curse's repo system, localization app, or any other features to be eligible for rewards. You just need to upload your addon there.