I'm seeing a problem...if you till the ground and reseed, doesn't that tear out the blackberries... and how do you get under the pear trees?
this might be a job for a roller to squish down the high places... Unclear what kind of soil you are dealing with... if done at JUST the right time when the moisture is RIGHT... I think you would smooth things out pretty well...
Net, however, is that you will NEVER get all the problem spots fixed.. and people can catch their shoes and fall simply from grass runners or blackberry runners.
So, suggest you ALSO think about a stabalizing help for her... some sort of cart with big wheels that she pushes and uses and has a handle to hold on to. She can put implements and produce in the cart, hold on to in going and coming across the pasture, carry a mobile phone with her to call for help if she needs it.
Is it is possible that there are favorite paths that you can focus your smoothing operation on and leave the rest?
I faced the problem of aging parents and need for assistive devices... I put hand holds in shower/tub, no skid strips in tub, got electric wheel chair BEFORE it was needed (makes a wonderful tool for moving somebody who has suddenly developed mobility difficulties..sprained ankle, etc.), got a 4 wheel push walker that they use both in house and outside... it requires thinking about the needs EVERYWHERE they go...
Good luck, she's lucky you are thinking about these things ... I was able to keep my mom going with no busted bones until her death at 92 this year from kidney failure.