2) I have written some support functions: mapM' and filterM'
Are they well written and generic?

mapM' is generic and already implemented: fmap
(Note that a Monad is also a Functor)

Except for when it isn't, which is really annoying because it ought to be. I just pretend it is, and then grit my teeth and kill a baby when I get bitten.

And mapM' is not fmap, but fmap.fmap (or liftM.map if you prefer).

filterM' is specific enough not to deserve any package
filterM' = fmap . map

Uh, I think you're looking at the wrong one.

filterM' :: (a -> m Bool) -> m [a] -> m [a]

I would write this as:

filterM' p = filterM p . return

And then probably just go inline it.

that I like more...

Are they already available in some package?
Can you suggest better names?
3) I find
(,) node `liftM` walkTree' path
not very readable.
Is it possible to express it in a more (not too much) verbose way?

I find it readable... It's ok IMO.

A bit ugly, perhaps. With the notation from Control.Applicative:

(,) node <$> walkTree' path

I would really like to be able to do a proper-looking tuple section though;

(node,) <$> walkTree' path

Luke