HI HB,
I'm not an expert and I'm not sure if what I'm thinking could work, but it's an idea.
What about to rent an excavator and a big pontoon, put the excavator on and simply extirpate those stumps?
My english is really poor so I hope you'll understand what I mean.
Something like this:
https://www.youtube.com/watch?v=lvj0ksYCSKE
