It's a step by step process. Container theory says that we not only have awareness of the immediate physical state but we also have awareness of ourselves as the observer, observing the container. To be able to do it, we have to have some awareness of ourselves other than in the immediate (duality). Is it possible? How do we actually do it? Most people who have had OBE’s will say yes we can and do, do it.
Ok so say I’m now able to watch/observe myself and everything in the container while still being fully present inside the container, aware of myself in dual states. That's a big first step.
You're a juggler and you now have two balls in the air. Initially all of your attention is on those two balls. If you shift your attention or get distracted oops you've dropped them and have to start again. But with practice you can easily juggle two balls, then three and more. It becomes second nature. You can have a conversation and juggle the balls without dropping them. You haven’t lost your mind in the process. You’ve gained access to more of it. So what's happening?
You've expanded your senses/awareness to include the balls moving through the air.
Once you've mastered the art of being the observer and the observed (detachment) and it's become second nature, the next step is to become aware of the environment of the observer.
You said, ‘All the spirituality in the world is useless unless it helps us live better practical physical lives.’ With ‘Container Theory’ you are presenting exactly the idea that begins to make sense of OBE’s. An OBE give us the ability to observe ourselves. They are one and the same.