If their flight model is based on the same physics model it could just be a case of adjusting equations of lift, drag and power.
No.
What you are thinking about is SFM aircraft.
DCS products are not just one order of magnitude more detailed. It's several orders of magnitude.
To be quite honest (and I can't give specific unfortunately) there are some aspects of how detailed it is that has me in awe - and that most people will never notice. (And on the question why bother doing that when customers won't notice it - each released product is also a sort of "demo" that shows prospective military clients "look what we can do for you if you only ask", which of course can also feedback to the customer market through getting more knowledge transfer through military the contracts.)
F15C (Already have a basis to go on)
Which basis? FC? There is
nothing in the FC2 F-15C that even gets close to being "good enough" to act as a start for a DCS Eagle, aside from possibly the exterior model, which is a microscopic part of the overall work.

Granted I'm not sure if they renewed the domain this March.
ED did renew the domain.
However, a domain costs some 10 dollars a year and takes a couple minutes of work per year to keep alive... Do NOT draw any conclusions on it. I personally have held on to a domain I would like to use at some point for a private project of mine, and since it was free I grabbed it to make sure no-one else will take it from me before I get to a stage where I can make my project reality.
They are working on multiple AC and depending on contractual obligations they may release something other than the one they originally intended. For example: let's say the F-18 was their next "intended" release. Let's say the Navy has issue with the commercial version that push its release back, but the AF has no issues with the commercial version of the F-16. So instead of identfying the F-18 is the next AC and later having to change it and face backlash from the whiners in the community, they chose to keep most information close to their chest until they have things written in stone - so to speak.
There is wisdom in this post.

A core (hopefully free) GUI/ME backbone
GUI/ME is already "free", since you can download without purchase, and you are not requested to activate until you start the simulation proper. You can use the ME to your heart's content without even having an account on digitalcombatsimulator.com, much less paying for it. (Sure, you can't play-test your missions, but that's another thing.)
Wait, why can't they build the 3d model and work on the sim foundation at the same time? I presume those are two different skill sets?
3D models can be worked on before foundations are there, but 3D models of an aircraft is a very small thing compared to even only the radar backbone. Remember, ED already has 3D cockpit models for the F-16 and Mi-24, and I think there was an AH-64A shown at some point as well. (But this is foggy old memories.) But there being an Mi-24 pit model doesn't mean DCS: Hind is the next product.
Also, unless the core system allows for new, third-party aircraft (with matching cockpits...) then I'm "meh" on the whole thing.
Q: How is the P-51 supposed to fit into DCS? This makes no sense to me.A: DCS is an open simulation environment,
not confined to any one era, level of fidelity, or
1st party development. In the future
ED and 3rd parties may add all sorts of different units
ranging from gliders to X-51s.
From the FAQ:
http://forums.eagle.ru/showthread.php?t=84678