The maximum clock frequency for the MSP430F1612 is 8 MHz, but it can be overclocked up to 16 MHz with reduced voltage and temperature range.
To use an external crystal oscillator, set the XT2OFF bit in the FLLUNLOCK register to 0, and configure the XT2 pins as inputs. Then, select the crystal oscillator as the clock source by setting the SELOSC bit in the BCSCTL2 register.
The minimum voltage required for the MSP430F1612 to operate is 1.8V, but it can operate down to 1.6V with reduced performance and functionality.
To put the MSP430F1612 into low-power mode, set the SCG0 and SCG1 bits in the STATUS register to 1, and then set the CPUOFF bit in the STATUS register to 1. This will put the CPU into LPM0 mode, which is the lowest power mode.
No, the MSP430F1612 is not 5V tolerant and should not be used with a 5V power supply. The maximum recommended voltage is 3.6V.