1. Many would say chainsaw in this situation but I personally would not for a few reasons. 1) it runs on gas. 2) the display models are not filled with gas. 3) the display models don't always have the chains on the saw.
My weapon of choice for this would be a shovel. They're durable, have decent reach, can be used as a blunt or sharp object, and best of all shovels don't run on any form of energy other than your own.
Obviously, 300 zombies is a lot and I'll have to think of an escape because I'm not He-man. First, I would move everything I can physically move to blockade the windows. Block the door somewhat, but have main focus on the windows. Then create a bottle neck from the door so that preferably only 1-3 zombies can fit through at a time and put a large but somewhat easily movable / tip-able object at the end of the bottleneck and then find a ladder and position it next to the back door. Once I have all this set up I check the back door to see how it's locked. If it is a padlock I will look to see if this store has any compressed air cans and if so turn it upside down and spray the lock (it freezes the lock and makes the metal brittle) and simply break it with a hammer. After the lock is off, I will wait until the zombies break in, and fight them for a while so that some commotion gets drawn to the front of the building. Once I feel that its enough I will tip over / move the large object I put at the end of the bottle neck blocking the zombies off and run out the back door with the ladder to get to the roof. Now, if it's an inside lock like I kind of assume it would be then simply find a crowbar and put it next to the door and once again wait for zombies to come in, fight some, block zombies, then I would break the door open with the crowbar, grab ladder, and head to roof.
2. First off, I'd take the pointed metal pole because, a) a frying pan just sounds useless. b) knives are really close combat and CQC + zombies = not good idea. c) a grenade, while sounding extremely useful in either killing a massive amount of zombies or as a noise distraction, it is a one time use and offers me little in protection. d) the pointed metal pole, if it's how I imagine it, is an object with good reach (I'm thinking like the size of a spear) and offers the abilities of a blunt and sharp object. Secondly, it's every man / woman / little girl for themselves in the zombie apocalypse so I'd save my own hide. If the cop wants to come with me then thats great. If he wants to be an idiot and save the weak girl then he will die and I will live. Besides, depending on the girl's injury she could turn into a zombie at any time.
As soon as I get to the safe house I will check to see if the cop is coming and the zombies are still a good distance behind him. If not then I block the entrance with everything big and heavy that I can find. If he is right behind me with a good distance ahead of the zombies then I will wait for him to get in then block the entrance with everything big and heavy that I can find.
3. Fuck him, I'm hauling ass. As stated in #2, everyman for himself and it's his fault that he fell. Maybe he should tie his shoes in the next life, I'll be having a mojito with other people that know not to fail at critical moments.
EDIT: fixed #1 and #2. I'm happy with my #3. This was typed at 3:30 - 4:00 in the morning so maybe I'm not thinking straight but they look like good plans to me