Gramatically they seem to be OK. The usage or word choice in the second one however is a little odd... just a little stilted to a native speaker. I'd suggest:
2. A great deal of that success, we are certain,
has been achieved [s]under[/s] thanks to your strong leadership
and [s]fine personality[/s] personal style.
"Fine personality" is a bit effusive. And it's not clear what about the person's personality the writer is trying to praise so it's difficult to choose a good adjective for "personality."
Of course these are sylistic changes so you can take them or leave them.
Hope that helps.