Re: Sad times
There was no possibility of rescue if the LEM could not leave the surface: he would have returned alone. Indeed there's a famous speech prepared by Nixon for that eventuality. Collins has spoken about the implications of having to do that, saying it would have made him a 'marked man for life'. I'm sure he would have done so however.
If the LEM ended up in an orbit which was too low due to some partial failure of the ascent stage then there were contingencies for the CSM to meet it in a lower orbit. There are some fairly nasty limits to that, not least that once the LEM has staged they can't last very long in orbit as they've just left a lot of their power (and air supply?) on the surface. There also has to be enough fuel left in the CSM to get home otherwise there's just no purpose in it at all: if you can't meet the LEM and get home better to get one person home.