Falling backward is only dangerous because of the vertical (too much upside down, not enough flat), and it is just as dangerous to land forward towards your head/neck (ala shiho-nage or koshi-nage) than backward (ala irimi-nage) on your head/neck. While height of the fall does contribute to the amount of damage you will sustain, just your own body weight slammed onto your neck is sufficient to cause damage with no height/speed - although I usually practice shiho-nage in hanmi-handachi to lesser the risk anyway.
For other falls I "snap the wet towel" as I mentioned in another post, keeping my hold on uke until they level out flat - then release them (or move to a pin) after they hit the mat. I don't allow them to go head first into the mat ever.
Last second direction changes are the same as stopping abruptly (the second most dangerous I mentioned). Uke is prepared for (and possibly half way through) a certain type of fall and then nage is halting/changing the fall. Uke is now unprepared and doesn't have enough time to react so the fall is erratic/awkward and generally chaotic.
I personally never change direction mid-throw for that very reason (it has always resulted in some sort of close call or minor to serious injury). Instead I simply grab uke and say, "wait!"