Hi Harshul -
1. Sure, you can add any metrics you are tracking. Session duration can be difficult to quantify as there isn't always an exit event. Metrics are arbitrary SQL, so anything you can query you can use as a metric.
2. You can do multivariate testing, if you mean like N number of variants. If you mean look at the results of A in exp 1, and B in exp2, then that can be done manually, but currently not from our UI