control playback speed; step through playback note by note

feature request one:
control playback speed as playback happens. (ie be able to speed up or slow down playback)

feature request two:
(this is the more complicated request, but the more useful to me)

i have to listen to what i wrote -- and slowly -- in order to see if i like it or if it "sounds right".
i can do this easily if everything is on one staff -- i just use the left and right keys, and everytime i press a key, the note will play. (the only thing i wish, now, is that i can have those notes sustain their sound for longer than a split second).

what i wish for is a similar ability across all the staves.

here's an example to make my idea clearer:
suppose i have a piece only composed of eighth notes, and note lengths that are multiples of eighth notes. (so no triplets or sixteenth notes).
` now, suppose i have the "manual playback step rate" at (say) "eighth notes".
` now, in manual playback mode, every time i press the right key (for example), the playback will step forward an eighth note, playing all the notes on all staves at that position, and sustaining their sound.

now, for a more complicated composition that includes (say) sixteenth notes, or any duration smaller than the "manual playback step rate" of eighth notes, then if i step forward by an eighth note in playback and this causes two sixteenth notes to be stepped over, then both sixteenth notes would be played (with the last one sustaining its sound).

this second feature request would really help me "hear" or "make sure" that the harmonies i choose are really what i want them to be.