1) Not right now, however you can do semi sequential and distributor as is, and by the time you have hardware built
Sean's XGATE code should be complete and tested enough to integrate into the main code. You could volunteer to test it in whatever state he has it, but that is at your own risk and you are strictly forbidden from badmouthing FreeEMS if you have any kind of bad experience with code that is not ready for general use such as that.
2) No, sadly no such feature list is available yet, however it would be best described as "less than the minimum required" or "not worth listing" at the current time. The priority is the rock solid stability of the core code, not features, for the time being, however, you should see
this thread to realise that if you hook something up, your priority is my priority, assuming that it is a reasonable request and beneficial to the majority of other potential users. Additionally, you can check out
the roadmap on the issue tracker to see some of the things that need to be done. Note, user requests and general features are in the ASAP section further down. They are generally easy and quick to implement, so it's not a big deal that they aren't there yet. Although there is a lot on that list, some of which look very important, the system is very usable now, I promise :-) BTW, "hardware dependent" would be a better term.
3) Correct, the OEMs have fuel quality concerns to deal with, and emissions standards to deal with, and very fine tolerances on everything. IE, they have to run right up into an unsafe state of tune and RELY on such systems to pull timing out if things aren't going well. This does NOT work well for very high output setups, ask any experienced tuner. It's also not necessary to get more than acceptable power and economy out of any given engine setup. If you tune correctly up to close to the limit and run fairly consistent fuel, you're going to be just fine. Once you've tuned an engine or two, this will become apparent :-) BTW, the newest hw design is planning to incorporate knock sensing, and I'm planning to use that as a development platform for it, so there will be support, but it MUST come after more important key things.
4) Very doable, just make sure you consult with me about what type of RPM pickup setup you're planning to run and make sure I have at least a week's warning to get it done for you.
5) Use the RavAGE pdf schematics combined with the TODO.md file to design your custom hardware. It is close to perfect and a great base for DIYing.
Welcome along! :-)
Fred.
Would be great to contribute. 
