Here's my take on the questions in this thread:
A remote start won't help the car run better or last longer in cold weather, but it can increase comfort and speed up defrosting the windows, which can be necessary sometimes. I don't think it's great for the car to let it idle to warm up, but it's not really going to hurt anything with a modern car. My car takes forever to warm up idling if it's below about 10F in the morning, but that's another issue (I need to check the thermostat when the weather gets colder).
A block heater will be more effective for warming up the engine efficiently. Of course it's not necessary for the car to start since modern cars are designed to work well over a huge range of conditions. But it will slightly reduce wear on the engine and save fuel by not burning it idling and by running more efficiently when it does start. The car won't be warm when you get in, but it will warm up fast since the engine starts out not as cold as it would otherwise be. The factory block heater for my car is about $50, and I might get one when it's time to change the coolant. Installing it requires draining the coolant. There are not that many mornings below 0F here, and I can't always park near an electrical outlet, so it might not be worth it.
I will use the floor mats that come with my car until they need to be replaced, then I'll get rubber ones. So far they are fine, and I found the best way to clean them is to bring them inside when they are dry and vacuum them with your household vacuum like you would a rug or carpet.
For the back seats, could you use a bed sheet? Or just place a towel on the bench cushion and hope the seatback doesn't get too dirty? The rear side curtain airbags usually come from the ceiling. If there is one in the seat there is usually a cloth tag that says side airbag, and it should say for sure in the owner's manual.