You can exchange single keys, providing you have access to a 2011/2012 US keyboard. The little siccors under the keys are different to the 2010 and earlier models. A topcase replacement with the proper american (small) Enter key would be the most expensive option.
Its a 4 point ZIF socket, but I don't know where to purchase one. Even if you get one, maybe salvaged from a busted board, it'll be still a !#*^ of a job to solder it on. You can always fit the keyboard sans backlight.
The superdrive is connected via SATA. There are companies selling drive bay kits, just google it. IMHO it is better to replace the startup disk with the SSD, put your system and apps on it and put a beefy data disk in the optical drive bay.
Disconnect the power cord. Remove the user access panel from the bottom of the computer. Press the PMU reset switch once on the bottom side of the logic board. Do NOT press the PMU reset switch a second time because it could crash the PMU chip. The switch is near the memory slot, next to the airport card. Wait ten seconds before you connect the power cord and keep your fingers crossed.
You'll have to find out wether the machine charges the battery at all. If you have access to a multimeter measure the charging pins. The two outer ones are gound, the three inner ones should read something around 3.58 V each. If that is not the case you'll have trouble with the SMC (more likely and more troubling) or the DC in (Magsafe) board which is rather unlikely since your machine powers up on AC.
I swap drives all the time and it always works. When you start up a foreign drive the boot time is a bit longer but you should be alright. I have a bunch of test disks with systems on that i use for troubleshooting and never ran into problems.