Photographs? Are you adding a gas line so this is the issue with the connection? I would not cut the pan as you asking for a source of failure along the glue line.

I believe you'll need a stacking kit which you can get from the manufacturer, I would think they'd have advised you of that. You shouldn't install, or at least run, a dryer behind a solid, unvented door. Be sure to allow enough clearance for the doors and be sure that whomever is doing the remodel understands what you're trying to do.

At first glance the floor drain idea sounds like a pretty good idea, but when you think about it, the only thing seperating the inside of your lovely home from a filthy sewer, is the U-bend water trap in the floor drain. No problem if you flush it regularly, but if you let it dry up you're going to be waking up to the lovely smell of sewer gas. However, this would work if it can be connected to another drain, and then have both drains flow into a U-bend trap. That way the other drain would presumably keep the U-bend closed off. I'm not a plumber though, so please treat this as speculation, not a suggestion. As far as putting the washer in a tub is concerned, I tend to agree with your sales person. The risk is that it will lower the friction between the washer and the floor, and your washer will start walking all over the place. It's true that a washer on a pedestal is going to weigh 250 lbs plus, but the problem is resonance between the washer and the floor, not that the washer isn't weighing enough, because these resonances are specifically caused by the washer being so heavy. So, if you have a vibration problem with the washer, it's specifically because the 250 lb weight of the washer is interacting with your flexible floor. Weedmeister's suggestion regarding manual single handle water valves actually has quite a bit of merit, because if you think about it, you realize that you're dealing with probabilities here. Let's say for aurguments sake that the odds of you having a broken water hose is 100,000:1, and the chance that you would forget to turn the hose off is 100:1, you can calculate the chances of you having a flood as 10,000,000:1, and you've effectively turned the remote possibility of a flood from a broken hose into something that's impossible for all practical purposes. The trick lies in being able to train everyone who uses the washer to turn the valve handle to the off position after every wash load.

the issue is that with new construction nowadays, the floors are not that strong. A stacked pair will weigh close to 300 lbs (at least over 200lbs) concentrated in a small area. Reinforcement usually means putting another layer of 5/8" or 7/8" plywood screwed directly to the floor joists where the washer is to be installed. Use exterior quality (waterproof glues). You can paint it, or tile it to make it prettier. The other thing you should have is a drain. If there is an outside wall, you should put the washer in a pan and run a pipe from the pan out through the wall. If there is no drain or you cannot install one, you should still put the washer in a pan just in case.