Yes 10-12 at the usual 1.25 quarts. I've done up to 14 pounds with using a lower water to grist ratio. Green Bay Rackers Mash Calculator.
Scroll down to the "can I mash it" calculator. It's a valuable tool to use to figure out how much grain can be hel in any sized mash tun.
I've been using my 5 gallon for 6-7 years and have never had a problem with 99% of my batches. The others, the rare barleywine I either borrow someone's larger cooler, or do a 2.5 gallon batch.
I've done 15 lbs in my 5 gallon rubbermaid water cooler. It seems like the holding capacity is closer to 5.25-5.4 gallons though. I mashed with a 1:1 ratio and topped up with correct temp water once I reached my mash temp to get a little thinner mash ratio. It worked out fine; I just had to be careful when stirring.
Depends on your sparging process. Fly sparging takes a lot longer than batch sparging. With a batch sparge after you vorlaff you can open it up wide open and empty it in a few minutes. Fly sparging can take 45 to a hour.