2) V$EVENT_HISTOGRAM view is for understanding distribution of wait times. For example, if average time is 8 ms it can mean that all events take roughly 8ms, or it could mean that half of events take 1ms while the other half 15ms, or that 99% events take less than 1ms while 1% of multisecond waits brings the average up to 8ms, etc. These are very different scenarios. In order to understand which you are dealing with, you can use V$EVENT_HISTOGRAM.
select event, average_wait from v$system_event where event in ('log file sync', 'log file parallel write');