Thoughts on Conjuration:
Here is a strategy I used for a summoner/conjurer BBEG:
1) Sudden Extend / Maximize Time Stop
2) Summon Fiendish T-Rexes
3) Laugh as the party gets wish boned
Okay, onto the Necromancy.
Answer to Question 1:
The first question I have to ask is, is the BBEG a divine or arcane necromancer? Sadly wizards tend to get the short end of the stick when it comes to being a necromancer. The biggest thing has to do with being able to create undead but not immediately control them. Clerics are just better at it. But there are options we can use for the arcane and since it's a BBEG you can kinda hand waive that away.
Do you have any preferences on themes for the necromancer? I personally like Pale Master and Master of Shrouds from LM. You can also take a look at the complete divine web enhancement class, The Nightcloak.
Pale Master gives you more and more immunities as you level up in it. Critical hits, stunning, free Undead Cohort, and you can punch people in the face so hard that they join your undead army. The undead armor is either going to be expensive (LM versions) or weak (MIC versions).
Master of Shrouds lets you summon incorporeal undead, starting at shadows and only getting better. Plus it stacks with your cleric levels for rebuking purposes.
If you go the arcane route, you can trade your familiar for an skeleton that gets stronger and stronger as you level up.
For feats, the corpse crafter line is good. Lich loved, while a bit icky, lets you walk about while your mindless undead ignore you. Lord of Uttercold and Black Lore of Moil are good metamagic selections. Tomb-Tainted Soul will help you if you're an evil cleric.
Finally, you can always go and make yourself a lich. Nothing says "Frustrate the party" when they have to go plane hopping to find your phylactery.
Answer to #2:
The rod of Undead Mastery is awesome letting you control 8 times your CL instead of 4.
If you don't have access to desecrate or unhallow, get yourself a dark skull. Be sure to get an altar in there too.
Staff of Hunting Spirits (CD) is kinda neat. Or a custom staff with whatever spells you think you'll cast most is a good choice.
If you're undead yourself then a cloak of turn resistance is a must have.
Answer to #3:
Undead leadership works similar to regular leadership except you get a penalty for attracting living cohorts/followers. Each undead has a LA/ECL for determining what you can attract. There's a short table on pg 32 of LM that should help. If you go Pale Master you'll get an undead cohort for free; saving you a feat slot.
One way to get even more undead is to have a wight cohort or one you've controlled magically. As your wight kills stuff they'll rise as wights under its control.
Answer to #4:
It depends.
Skeletons are faster and can do more in a turn. Zombies are slower but they can fly if they had wings.
A skeleton thoqua or rhemoraz is pretty awesome.
A hydra zombie or most outsiders-turned-zombie are just mean.
So really, you have to look carefully at what each creature loses and gains when you animate it. Sorry this isn't that much help.
Answer to #5:
What do you mean by random spawning? That you don't want your army spawning more undead? If that's the case then LM has an alternate rule on undead spawning. It allows the creating creature to make an Int check to forgo the creation of a spawn.
Mindless undead aren't going to be good for tactics other than "go there" and "kill that". Zombies can make good damage sponges.
Intelligent undead however, can and are just as clever as the living. So anything that you could expect the town watch or the king's army to do; a group of ghouls could reasonably do too.
Answer to 6:
Spells to consider include Ray of Enfeeblement, Shivering Cheesy Touch, Energy Drain, Wail of the Banshee. You can even use necromancy to try and stack fear effects on the party. Your goal is to use your necromancy to debuff the party from a distance while your ten headed hydra zombie eats the party.
If you have a large amount of low HD zombies or skeletons, load them up with Unliving Weapon. Nothing better than having the party blow themselves up.
Other Advice:
The biggest problem you'll run into is that if the party knows that the BBEG is a necromancer they are probably going to prepare accordingly. Death Ward, Soulfire Armor, Hero's Feast, Spawn Screen, feat and weapon choices can seriously neuter your offensive capability from your spell selection and your undead. An undead whose primary danger is negative levels is not a threat once your party pops off Mass Death Ward. So have a back up plan of some sort. Either through dispelling their magical enhancements or by punishing them for having it up (Reciprocal Gyre). Blackmail the party into doing ... something. (Leave this place or I'll have my ghouls eat your loved ones) Or set yourself up as a load bearing boss.
