1) Yes, you need to wake the PSoC 5LP and set the boost converter to Active mode to refresh the regulator power supply. There is no specific wake-up time, but not recommended to give large Wake-Up times. CTW (1KHz ILO) will be good enough.
2) There is no need to wait till boost is active. But, you need to save the clocks before Sleep Mode entry and restore it back once the chip wakes up. Many of the clocks are not ON in Sleep. Hence you need to save them and restore it back after wake-up so that their original configurations are restored.
3) Yes, theoretically this is possible. Ensure that VBAT > 1.8V. Also, highly recommended that PSoC is in Sleep mode during this time.
Quick Update re:
Switching stopping / restarting boost on the fly seems to work well. I do not see how you can do this in sleep mode as the core has to be active to execute the Start and Stop commands .... so I am currently doing it in active mode and not seeing problems.
Was the recommendation really to issue the start boost command and then immediately afterwards enter sleep? If so what is the reason for this? While the external supply voltage will step the internal core voltage is supplied by the core regulator which should remain steady at 1.8V considering it has a 1uF capacitor on it. Or do I have the wrong end of the stick here?
2) I implemented 1) & 2) and it worked okay (but a 1KHz wakeup is much too power hungry for my battery). Just to kick the boost and go back to sleep you don't need to restart the system clcoks (which again saves quite a bit of power not having to do this). However approach 3) suits my use case far better from the point of battery life .... I am data logging and only need the higher voltage very periodically to store my results to an SDCard.