Hi, not sure why you need to use another metronome to go slower that 40bpm. If I want to play at 30bpm I just set it to 60bpm with the accent on two and play on every other beat! I sometimes go right down to 15bpm when learning something new. Again I use 60bpm and just play every fourth beat.
When practicing chord changes at such a speed I would lift my fingers off on the first beat and put them down on the second beat. You probably wouldn't want to do this at song tempos, as then you would want each chord to ring out for as long as possible before switching to the next chord.
Works for me
As for the subdivisions? I have this feature on other metronomes but never use it - too many clicks for my liking. Doesn't mean others won't of course!
My opinion is that I need to learn to feel the subdivisions myself and not rely on the metronome. Besides if you wanted to have say triplets at 60bpm you can set the metronome to 180bpm with the accent on 3. Yes you then lose the accent to mark the start of the measure, but the basic timing is the same.