Re: sink hole
As long as the sink hole has been created by the decomposition of stumps, there's no harm, no foul to fill with dirt/top soil. I would not recommend gravel or rock of any kind, partly because this is your garden, partly because that's not the way to fill a depression in the ground.
If the sink holes are caused by erosion, you'll have to determine the cause of the erosion and remediate that first, then go about filling the hole.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!