How much is customizable in a WordPress multi site with custom domains?

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.

If I was just starting out, I'd consider learning Meteor (https://www.meteor.com/). It's just entered version 1.0 and after working with it for a little less than a year I do have some issues with it but it still makes for a very solid framework that gets you up and running very fast. You would only need to learn Javascript, and you can slowly work your way towards nodejs from there (which Meteor is based on) if you want to, or you could get the basics down and focus on learning design if you prefer.

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

Usually Programmers are only slow when they don't know how to solve a particular problem. So they will spend a lot of time researching and a lot of trial & errors to solve a problem. It is important that before you engage a programmer on a project, you break down the entire project into simple, easy to understand modules. Let him give you an estimate of how many hours he will require to complete each of the modules. Example: a typical site will have a login module, registration, My account, profile etc. So let him estimate how much he will require to do the login. You can go even detail here. (e.g. how much extra time if you were to implement Facebook/Twitter Login?). Once he start developing, track his progress closely and make sure he is following his given timeline. If he goes over his budgeted time on a module, talk with him and see what went wrong. It is often seen that they may be wasting their time on something very insignificant that you may have asked him to implement, but you can totally go by without it too. So by understanding what is taking longer time, you will be able to prioritise things better. You definitely need some tools to get this done. Google Spreadsheet or Excel works just fine. But if you don't mind spending a few bucks there are many agile project management tools that you might look into. Here is a list, google them all and sign up for trials: * AgileZen * Agile Bench * Assembla * AssiTrack * Blossom * Basecamp * Breeze * DoneDone * Eidos * Fogbugz * GreenHopper * Jugggla * Kanbanpad * Pivotal Tracker Or the reason why he is slow can be purely non-technical. Sometime your developer may don't share the same level of enthusiasm as you about the idea that you are working on. They often don't often see the "bigger picture" (since you don't share everything with them explicitly). If you can somehow get them excited about what he is a part of, it will work like a drug :) He will work day and night without questioning you. But you need to work equally as hard as him. The moment he sees that you are the boss and he is just the guy doing work for you -- his mentality will shift from being part of something to being the low paid developer. Ultimately its all about motivation and making him a part of your venture. After all he deserves it, if he is really playing a crucial role in the entire development.

Be careful when simply wrapping things into a mobile app. This can backfire on you and you could be presented with some very harsh edge cases. PHP isn't going to be encapsulated so much as the HTML/CSS/JavaScript. So keep in mind your mobile application could likely be in a position of requiring an online connection to work (because it must interface with your existing hosted WordPress site). This could also mean your hosting solution needs to be evaluated to ensure you can handle any increased traffic (and those traffic patterns could be different when coming from a mobile app that perhaps loads things the user doesn't see right away, accesses content that may not be cached, etc.). You want to ensure your server doesn't go down because then your mobile app would be "down" as well. That said... Things like Phonegap (web views) are a wonderful idea for utility apps because the performance is good enough for those (and hey even some games) and they end up being easily ported across many mobile operating systems. I would look into Phonegap, Appgyver (a new and totally awesome one because parts of it will utilize the native OS and your app will feel more responsive), Appcelerator Titanium, and perhaps even the new Famo.us one. Also take a look at the Ionic Framework for some further ideas about mobile UI and what you can do with these web view style apps.