All good tips, main thing is get them when they are busy!
Naughty time busy
munchy time busy
chilling time busy
As said if they spook they will tend to come back if you stay still.
Feeding BFs time at a flower depends on the floret density and how often the flower has been visited. Single floret flowers, they may feed for seconds, densely packed floret heads they may stick around for minuets.
For lenses I suspect anything can work, I tend to find the extra reach of the 180 macro helps unless it is the sort that is low in the grass then the shorter 100mm is better.