There's a newer rulebook available online (easy to search and find) that is the 5th edition from 2008. In keeping with a tradition of bloat, it is an even larger pdf file.

I've you're really keen there's a set of every rulebook that came with the game going back to the Realpolitik days in the 50s at the Diplomacy Archive.

http://www.diplomacy-archive.com/diplomacy_rules.htm

One day I'll get round to reading one of them

You cannot dislodge your own unit. The adjudication system (at least in classic) is very robust and has no bugs.

Here's a list of different adjudication scenarios for future reference:
https://webdiplomacy.net/datc.php

specifically, it's test case 6.D.11

Hi,
I tried attacking my own fleet in a game with a supported attack (resulted in a 2v1 situation) and my losing fleet did not have to retreat, it is like I bumped into myself. My goal was to retreat the losing fleet in an enemy empty territory, which was certain to happen due to the current unit position.

I wondered if it is a bug or if I misunderstood that you could attack your own armies/fleet