I don't have much experience with AIX or HP-UX, but Solaris seems very good choice for those requirements.
1. If you will go with Solaris 11 Express, then standard Solaris development tools like Sun Studio are available, as well as GNU tools (gcc, gmake etc). Dtrace is great for debugging misbehaving applications, but it requires some time to learn.
2. I think any of those three systems pass this requirement.
3. Read about Solaris zones. It has all the features you are talking about.
4. Dtrace is the answer for pinpointing the problems. As for documentation former docs.sun.com can be found under:
Oracle Secure Enterprise Search which is amazing Solaris docs collection. Solaris 10 and 11 documentation collections can also be found here:
Oracle Solaris 10 9/10 Release Oracle Solaris 11 Express Information Library 2010.11 Release. Great community help regarding Solaris can be found here.
One more thing that you should consider is ease of running the system on workstation hardware. Solaris can be easily installed and used on regular consumer level x86 PCs, while AIX and HP-UX are restricted to POWER and IA-64. I guess you won't find many workstations running this kind of processors