1. Yes, First VM will start and shutdown as per the order. however, you can modify this behavior using startup and shutdown delay timers
2. both are a diffferent scenario.
HA is for failure scenario
Startup order is for maintenance activities
Let me know if you have any specific scenario that both the event occurs at the same time
3. If the VM is vMotioned the VMs will be moved to any order startup mode.which means the VMs will be automatically powered on along with the host but it will not follow the sequence