Yes, you can install the kit without the drain if you'd like. Just remember to check the can often for the first thousand miles until you know how much blow-by your engine produces.From the pictures, it appears the catch can is able to be installed without the drain kit. Is that true? I'd prefer to just empty the can.
As for the debate over warranties, etc, the same can be said for any modification you make to your vehicle while under warranty: The dealership still has the burden of proof that any failure was caused by the aftermarket part - they're going to have a hard time proving that your transmission went out because of a catch can. That burden of proof can be pretty light for some dealerships - our suggestion is to get to know your local dealer and learn how modification friendly they are. We've helped customers fight warranty claim denials due to catch cans in the past, usually all it takes is an email to corporate. But again, YMMV.
Oh, and keep your receipts for any oil changes you do yourself - it's not really "proof", but it can help