What's a good service to use for visitor referrals, i.e, a contest-system that gets visitors to your site in exchange for "entries".

Not totally clear on what you are asking, but if the questions is; does giving out a referral code to an existing user in hopes that they would refer another work? My experience (largely in B2B software) is not all that well, at least not without some sort of incentive. Even if your user is super satisfied with the product/service you are providing, simply giving them a code to give another person doesn't necessarily drive them to make the handoff. Now, two things. First, if you either incentivize the existing user with say a discount on his next bill or a free goodie, then he'll be more likely to do it... Even better, if you do that, plus give the referred user some kind of benefit, like a discount on his first bill, free trial or other goody, then it can work rather well. Second, all that said, know that referrals in general are gold. You should test and do whatever you can to get referrals. Generally @Leads360 we found that providing really high quality customer service (more so than even the best product) lead to referrals. To that point, our sales people worked in tandem with customer service in this way. Whenever a CS person realized they gave a great customer experience they would let the sales person know and they would then reach out to that customer while still warm from the nice touch and simply ASK for a referral. I was always surprised when we could get referrals simply by asking. I like to stick with 1 referral at a time, just ask for 1 person to be email connected with, don't overwhelm them with the statement "hey, can you refer your friends and colleagues to us", be specific. Something like "I see on LinkedIn you're connected with John B from ACME corp, I'd really like to speak with him about our product, would be willing to make an introduction for me".

Well first off I wouldn't recommend trying to power your client's database with the standard WordPress database instance. They can use WordPress for the website all they like. But with millions of records like that, you really don't want to move those data in with WordPress. For that matter, you're probably gonna want a dedicated database server to answer those questions, if the client doesn't already have one. You might also need to investigate something like Elastic Search, depending on what your search requirements are and what sort of database they are using (pay close attention to whether or not it provides full text search ability). Beyond that, and somewhat obviously, you're also gonna want SSL on the website and to make sure you're up to date on federal and state privacy requirements for data like this, as well as best practices for securely transferring data over the web (port configuration, ssh keys, etc). The privacy concerns would raise a flag for me. If those data have anything personally identifiable in them - which they very probably do - then you're really gonna want to make sure you do your research on security of personally identifiable information. Government specific privacy /security /auditability requirements may also apply, so double check that too.

You are comparing apples to oranges. WordPress is a platform, Ruby is a web framework. If you are looking to develop a completely custom platform, you could certainly do it in Rails, but you could also write it in PHP (like WordPress). I'd do some digging and see what is out there for pre-made software first before you build something from scratch. I'd imagine there are already plugins that would do this sort of thing for Expression Engine, Joomla or WordPress. No need to re-invent the wheel!

Yes. https://docs.woothemes.com/document/configuring-woocommerce-settings/

There is a lot of points in your question which unless someone has the exact same setup as you - I don't think you will get the answers you are looking for. I believe you should limit it by asking yourself: What is the ONE thing the MLM plugin I am looking for can do so that it achieves the maximum amount of result to my business by using it? IMHO - it would be the ability to set commissions for the products you sell and award commissions to those affiliates that actually earn you sales. All other features, while nice, are ancillary features that from the perspective of the affiliate - don't really need in order to be successful. In that regard - I would suggest https://affiliatewp.com/. When it comes to running affiliate programs on WP sites, it wins out as the best in my experience. Hope this helps! Glad to assist any further if you need help.