Yes, the floor and rainfly seams are heat taped to prevent leaking.
Be sure the tent is clean, clear of debris, and completely dry. Storing your tent wet can encourage mold growth that will ruin the tent.
It depends on the tent, but 2,000mm is the lowest water proofing standard we use, which is still higher than a large majority of other tents in the market.
All sleeping bags 90" x 39" and smaller fit the Outfitter XXL Quick tent best. However, the Celsius XL 0°F bag can also fit in the tent with a little bunching.