Can I measure time by counting frames and trusting the '240 FPS' of my iPhone 7+ slow motion?

An easy, but not 100% accurate way of ensuring this, is simply to right-click the video in Photos on your Mac and select "Get Info". This will show the average number of frames per second over the whole video. This is usually not exactly 240, but extremely close.

Obviously as it is an average it can vary very slightly from one range to another within the file, but for most usages this is just fine.

If you want to be absolutely precise, you'll need to look inside the HEVC file itself, to see exactly how many frames are in each second in the file (i.e. look at every second by itself - not doing an average over the whole file). This requires you to be proficient with video file formats at a programmer level.

if you can start a stopwatch and include that in the frame of the slomo video you can get a pretty decent approximation of the time.