1. You would need to download the JBossTS product, in the bin directory there is a script called start-transaction-service which starts the JTS in standalone mode. But this configuration is not supported so I'd advise you to simply start it running on Server1 inside the AS container instead.
2. JTS supports transaction propagation so you don't need to use a centralised transaction manager.
3. I'm not familiar with the OFBiz integration issues. But if you need to remotely start transactions from Java then there are examples of how to do that on the WIKI. If you need to start transactions from C/C++ then, since the JTS uses CORBA, we have solutions for that also.
Just out of curiosity, can you explain the reasoning that leads you to conclude that putting your entire cluster's transaction load through Server1 is 'truly scaleable', much less performant or resilient. Aren't you just introducing a bottleneck / single point of failure?