1. It depends on how old that DME is. I believe it has a shelf life...especially if it's been opened previously. I'm not sure how long it lasts as I brew all-grain now and don't keep the stuff around.
2. Yes, you can mix extracts.
3. You'll need about 6oz. of DME for a 2000mL starter.
4. Stir plate takes the deal. Anytime you can make a starter with a stir plate, take the offer. I treat my starters just like I treat my beer. Temps may not be as big of a deal, but why get into nasty habits?