Borders, welcome to the forum!!! Congrats on planning your pond now instead of finding us to help solve mistakes after....many of us found the site the latter way!
I don't have much experience with stump grinders other than they just remove what you would see, in a field. They won't get anywhere near all the roots that are hidden. The wood chips, however, I think would improve the soil for starting grass seeds on disturbed areas outside the pond pool area.
For your best chances of full sealing, the entire pool area will need to be stripped (saving topsoils for final gradings) a foot or two deeper than designed, and then quality clay is relayed and well compacted in 6" lifts to get your final under pool grades, and to tie into the dam core.
After the bowl is made, well compacted and sealed, you can get creative building trenches or underwater islands for structure...Root wads make an awesome place for fish when pushed up and the root wad is still filled with rock and soil as ballast.
I guess after all that was said, I'd excavate or doze out stumps for reuse in the pond....grinding or even excavating a bunch of stumps will also add LOTS of equipment time and fuel expenses....
