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You're missing one stat roll, but if it's as bad as your current rolls, you legitimately qualify to reroll stats, as per the standard ability generation rules of the PHB (no stat above a 13).

Your totals right now are 13, 7, 13, 10, 12. If you manage to not roll above a 13 on your last stat, you'll get a reroll.

And yes, you should roll for every stat. You're getting to forsake two of your ability scores - why wouldn't you chose to roll a total of 6 so that you can dump the lowest ones?

Edit: Also, rereading the DM's first post:

You should generate 7 stats, drop the lowest of those, and you still may get to reroll if you generate no ability higher than 13. So, in other words, if you roll your two additional stats and get two 17s, you can just ignore the 7, put the 10 in CON (which disappears), and the 12 in STR and be really well off.

Again, as Rock and the DM said, if you roll your stats, you can't then choose to go to point buy.

As for your Charisma casting stat question, there is the Dread Necromancer class from Heroes of Horror, that would use Charisma as its primary casting stat. Alternatively, you could use the Sorcerer and just take appropriate necromancy spells.

It's a spontaneous specialist class, like the Warmage and Beguiler, that focuses on necromancy spells and gets handy negative energy abilities. It's Charisma based for a casting stat and gets wizard and cleric necro spells on its arcane spell list. It can be found in Heroes of Horror.