for getting the .25u dose - have your sitter draw up more, say 1 unit, or something slightly less. then try "screwing" the plunger to work it down to .25u one drop at a time. sometimes you can have more control using the plunger that way. when looking at the plunger, take note of the "air space" between the plunger and the lines.
you can also count drops. that's how people do less than .25u, but perhaps it would help here as well. draw up an exact unit, then squeeze out one drop at a time, counting the drops, until you get to .25. make note of how many drops you squeezed out and then repeat the process the next time.
you'll have to use the insulin for trying the drop method because the "thickness" of liquids varies. fortunately, he's on a small enough dose that you're not going to miss the insulin!
regarding the hypos - this is hard to say. we are very data-driven on this forum and we test so we know where the cat's BG is.
low numbers are not necessarily hypos. i've had punkin show up in the 30's when i've tested, but i give him high carbs right away and he comes up. we treat numbers that are below 50. a cat can't become hypoglycemic without an injection of insulin. as i said, most cats will want to eat if they get too low of blood sugar. that's why i was suggesting leaving out plenty of regular food.
i think dyana gave you the links for hypos for your petsitter. everything would depend on how low Kitty got and the timing related to the petsitter's arrival. Kitty seems to nadir fairly early - perhaps around +4. if your cat sitter were to arrive and find Kitty in distress, she should rub karo syrup on his gums or administer it rectally and get him to a vet. that would be an emergency.
this is why we're suggesting caution. perhaps someone else has a better answer for you than this.
). I will also leave extra LC canned food. 
. I think it is likely one drop of insulin. I left a note saying in case they are in doubt, less is better.