A) Evil lists tend to be more powerful then other lists, but in general if a person fails rolls by large amounts it does not matter how high a level they are very the spell.
Note: A long time ago there was a House Rule that talked about changing they way the base rules worked in saving throws but I do not remember what they were called or where they are now. You might do a search to see if you can find the thread on the ICE Forums or at The Guild Companion.com to see if it was there (if it was not there then it might have ben in the RM Express Additions, but I do not have access to files on a old dead computer so I cannot help there).
But in general after looking at the rule, if I remember correctly it was something I did not see a need for in my game but maybe it would fit yours.
2) In general in my game spells that give effects such as possession or damage need the corresponding spells to remove that effect, so exorcism for possession or the appropriate healing spell for damage, ie Dispel Magic does not work for many spells that cause an effect but are not an ongoing magical effect, especially spells that cause an effect and then the spell part goes away.
In the case of possession spells, then IMHO the spell opens up the target to possession and then the spell (magic) part goes away so Dispel Magic would not work. But you game may be different.
How you rule on this issue could have a huge impact on your game and may require you to classify different types of possession (ie those induced by spell, by magical creature with a spell and by creature with an magic ability or a creature with a non-magic possession ability.
In general (again sorry) level is a good indicator of a lot of things but it is not the end all be all in my game and their are limits to the phrase "But I am 20+ levels higher so it should work this way because my high level makes me special".
If this is a problem for you I would talk to your players and find a rule that works for you and them and test it out to see what changes it has in your game.
MDC