We are currently designing a system which will use a relay, a MCU and a sensor (powered in 3.3V with 100mA max current consumption) + a few LED (2.2V type). it will be powered by a 9V battery

After inspecting a similar product we find out they use MSP 430 from TI which use 3.3V, and a relay HK4100F-DC5V which supposedly work with 5V DC (datasheet http://img.ozdisan.com/ETicaret_Dosya/445413_4369639.pdf/) . I read in this datasheet that coil voltage can also be 3V.

I need to choose the PMU which is going to be used to power this system.

1./ Do i really need to use 3 LDO/ DC-Dc to power 3.3V,, 5V and 2.2V or can I group all of those under a 3.3V power rail ?

2./ How do I determinate the current required by the coil of the relay ? Do I just take the voltage applied to it and divide by its resistance?