Will Google analytics track if instead of using the 3 mandatory variables (source,medium,campaign) I use just source?

It all depends on whether they are all selling the same thing to the same persona. If yes, then combine all the campaigns into one adwords account, which feeds into one analytics account. That way they can get feedback on the effectiveness of different keywords, ads, etc. more quickly (because there will be more data, from all the different websites, all in one place, for a particular keyword). If they are each selling different things, or to different personas, then don't combine their accounts, because it will just make things confusing and not useful from an analysis perspective.

If the transaction happens on the same website, as the test, you don't need to do anything. The user has a cookie that attributes the version of the test to the revenue tracking in your testing software (Optimizely, VWO). Once you connect the Google Analytics with testing software, you should be OK to go. If this is not the case, then you need to setup two email subscriber lists, one for each version of the test. Then manually tag email links in the emails to attribute the ecommerce revenue directly to the version of the test.

Google Analytics is the best free tool out there for this. Make sure that you are focusing your report on conversions - how many/what percentage of website visitors are taking action (filling out a form, buying a product, etc.). If you're interested in paying for help, you should check out Moz or HubSpot, but Google Analytics is your best free option for web reporting. They also provide some social reporting - which social networks are driving traffic to the site and which network referrals are leading to the most conversions. Good luck!

Do you have any interest in maintaining the domain? Is it getting regular traffic? To maintain that, you'll want to get some relevant content on the domain as quickly as possible. From there, you can decide what you want to do with it. If you just bought it for nostalgia or because it belonged to your competitor, and have no intention of doing something it with, you can do a redirect as the Google rank really won't benefit you. Let me know, -Shaun

Yes. While Experiments reporting is session based, a user is kept into the test, seeing the same variation the whole time, so when they convert, they will be attributed to the test variation they have been seeing. This is limited to the same browser on the same device, so if the user changes devices (20% of the time for average eCommerce sites by my estimation) or clears cookies (1%) or changes browsers on the the same device (?), than Experiments (and nothing else legal) can track them. Hope this helps. PS(I suggest you look at other tools, as Experiments is the last tool I would recommend right now).