furby,
Without an all-movable tailplane it'll be limited to subsonic speeds. My guess is approx .93 mach unless it has a flutter issue prior to that. All movable horizontal stabs are complex, typically require some sort of pitch stability augmentation to prevent over-controlling at various speeds, and if they fail then the stab tends to move to a "flat plate" position instead of to a streamlined position like you get when a normal elevator control rod fails.
It's MUCH easier to just go with a conventional elevator and give up high transsonic or supersonic speeds. It makes engine inlet design simpler too.
FWIW, the BD-10 crashed because the horiz stab suffered flutter at high speed and ripped the back of the plane off. Bad thing to happen IMHO, and I suspect that if they'd had a subsonic target top speed, it may not have happened.
Yea, it does look like a modified T-38 to me too. But there have been a lot of people saying that the T-38 modified with a new, slightly larger wing, would be a better trainer than the current model. Most front line military aircraft are waaaay easier to land than the T-38 so I personally think that a T-38 with 10-20%ish greater wing area and limited to .95ish mach would make for a fine next-generation advanced trainer or even a civilian hotrod. But that's just my opinion.
Regarding the twin tails, the T-38 gets away with only 1 tail because it has an active yaw damper system. The yaw damper is disabled/non-functional in many USAF T-38s but those specific aircraft have operational limits placed on them to avoid problems that the yaw damper was supposed to fix. My guess is that the javelin team found that they needed the twin tails to avoid having to go with a complex yaw damper system. The T-38 without the yaw damper probably couldn't pass current FAA certification requirements.
