Gravity there is so low that something like a light push may be enough to launch. If anything used to examine the surface makes contact and presses (drill, digger), good-bye lander.

Firing the harpoon would also be enough to launch, but at least the hook would grab the surface and pull the lander back down, giving enough pull to drill viable long-term anchors.

Are there any forces in effect as it gets closer and swings around the sun that might cause it to separate from the comet if there is no further attachment by then?

Absolutely! If its attached, its via surface ice. That ice will all melt on close approach to the sun. At some point the probe will drift away.

Of course, I should have deduced that. Is the harpoon expected to go deep enough to prevent this from happening?

Newton’s Second (edit: Third) Law. It wants to extend a drill bit into the surface.

Third?

…oops (apropos username)

It will dig into the surface anyway so it will anchor itself even without the harpoon, if it's on the surface of course.

See this video that explains how Philae foots work: https://www.youtube.com/watch?v=-77-Z_DHTlY#t=84

ESA says it landed gently on the surface: https://twitter.com/esaoperations/status/532575615527170048

It could actually, since there's already outgassing on the comet.