At the moment, no. The drivers for the integrated GPU work only when the GPU is integrated directly to the memory controller, whereas external GPUs are used over PCIe. The drivers are incompatible. There are some arm64 drivers at some download URLs, but those are intended for data warehouse arm64, and are unlikely to work with a Jetson. I’ve seen people ask about this, and I suspect NVIDIA is considering it, but I have not heard of such a thing being implemented (yet).
First of all, nvidia-smi is used normally. I have not tested the tegrastats command.
After starting in this way, the integrated GPU will not be able to use graphics mode, it can only work in character mode, just like a dual graphics laptop.
For battery power supply, the use time depends on the amount of GPU calculations on the one hand, and on the other hand, it depends on whether the extended power supply of the graphics card is provided by batteries.