First of all, the Rescue mission does not work in Marathon 2/Infinity, and I have no idea if anyone's bothered to fix it in A1. Aside from that, I think your idea of the way objectives work is essentially correct- if you click more than one, they must all be satisfied to get the Finished terminal sections.
How to set up a level to detect a fraction of some group of switches, in any combination, being pushed... hmm.
Off the top of my head, it would go something like this: Each of the "save a fraction" switches would be rigged to move a multistop platform through 1 step of its motion, starting at fully extended and receding into the floor. There would be a monster trying to shoot through it at a switch, placed at such a level that once the multistop platform reached, say, 20% of its original height (80% of the switches have been hit) the monster can shoot the switch. This last switch would be what decides whether or not you've saved enough switches, you'd have to make it permanent somehow (by making whatever it does irreversible or removing the monster shooting at it). To have the final terminal be able to display all required states (still enemies left, success, failure, and "still enemies left but you've already lost too many switches"), you'd need to use a multilayer terminal, so the final switch could simply swap the lights and you wouldn't need to use any map objectives besides Exterminate.
