But if the tube came out of there, it shouldn't be too long to go back in.
I'd try to figure out what's going on before I start cutting things.
The posts should be marked IN and OUT on the keg. Is the diptube on the correct side?
I can't imagine there would be a significant difference in how it fits from one side to the next.
Rather than pushing it down, try moving the diptube around from the inside of the keg. Maybe that will help.