Some changes have been made to the way thieves get their "points" for
practicing their skills. To some extent now, the amount of "points"
you can use will be based on your level. Some changes have also been
made to the "point value" for various skills. Existing characters
will have their points re-calculated. You may find yourself able to
practice a skill now you previously could not. You may find yourself
unable to practice skills you thought you could be able to. No
existing characters will lose any skills as a part of this
re-distribution of points.

What with the thief forum gone; is there somewhere we can get a general idea about the thief paths and how these points work? I'm curious, for instance, if a thief goes fully in one path, how many points/skills can he learn from another path. Etc.

Or is this still a mystery to the masses?

My post above is about as informative as I care to get about the
subject. You can play a thief and see what you can do and cannot do,
or you can ask a thief in game. Generally speaking, however, if you
fully complete one path, you'll be able to dabble in other areas.
How much you are able to dabble depends on the strength of the path
you finished, and the strength of the skills you are dabbling with.
I've now met my dabble quota for my lifetime.

As it stands, does one have to complete a path to get skills from another? There are many skills I'd rather skip in most paths, but things I'd like from at least three. Can I pick and choose freely?

You do not have to complete a path to learn the first skills of
another. You cannot, however, pick and choose what skills from
what path you want. There are prerequesites within the path.
For instance, you must know plant to learn bribe mercenary.