1 and 2 covered well by DakaSha
3. When hitting an opponent with Y remaining hit points for X damage you have an X/Y chance of causing an affliction. So If they have 10 hit points left and you hit them for 3 there is a 30% chance of an affliction
The death bless increases the chance of affliction, so if you had a +100% chance of affliction (level four death bless), you'd have a 60% chance in the above example.
4. It really depends what you are after. Your best bet at first is to look at the strategy guides in the Strategy Index, most of them have research recommendations. The generic choices first up are often those that allow remote site searching - Thaumaturgy 2, Evocation 2, Conjuration 3 (and blood 2 if you are heading that way, though there aren't many useful blood sites).
After that you need to figure out what your strategy is
Edit:Oh, forgot - with regards to Pretender selection. It's the most important decision in the game, sorry there are no easy answers
Again reading a few strategy guides will give you a good idea. It might be worth your while picking a starting race to get the hang of and follow a guide or two for them at first. Once you know them better you can improvise yourself
I advise against trying to learn too many different nations and strategies too quickly, get one thing down to a reasonable level then move on. You can easily drown in it all otherwise.
Edit2: One important thing to know is that most PBEM games run with the excellent Conceptual Balance Mod (CBM) by quantum_mechanics, currently at version 1.5 and located at
http://forum.shrapnelgames.com/showthread.php?t=43120
The devs don’t balance the game, they just add content and let users mod balance, which CBM does very well, mostly through modification to research levels and costs of spells, but also units. So if you are at the beginning of the learning curve I'd recommend just stick with CBM, it is more commonly used than the vanilla game by far and switching between the two can be confusing as some things change drastically