Just guessing. That warhead weighed over 8,000 pounds, so having an adapter buckle would definitely be bad news.

I did a little searching and couldn't find out what those cylinders were for. As best I can determine, the Titan II didn't have a post boost vehicle, so those probably aren't auxillary propellant tanks. They may have some other purpose like holding compressed gas or something.

I don't know how extensive the instrumentation would've been back then. The technology just wasn't all that well developed and they had trouble getting radio signals from the manned spacecraft during reentry, so it might've been difficult for them to get telemetry from a RV. Missile RVs are designed for a very high speed reentry, so they probably get a lot hotter than say a Gemini capsule. Back then, the engineers probably would've just added enough ablative material to make sure it didn't have a burn-through. They probably would've used radar data to determine the stability of the warhead during reentry.