Computers, phones, and devices that are not currently logged in with a given Apple ID and specifically with the service Find My Mac / Find My iPhone enabled will show up as "Offline" in someone's iCloud account etc.
So, basically, if both of you have the same computer in your iCloud accounts with Find My Mac, only the person who is currently logged in can track it. If neither of you are logged in no one can track it. The only way he could track it at this point is if you knew his Apple ID and password and went into System Preferences > iCloud and set-up Find My Mac for him. It is up to him if he wants to keep a line in his iCloud.com menu under Find My Mac that has the "name" of the computer that was retrieved the last time he tracked it that will perpetually search and always fail to find your computer, and just say "Offline". You can't change that for him.
You're safe. With the computer wiped, his user account isn't present, and that's where the iCloud account data is stored. You do not need to track your computer to "block" him from doing. It's just up to you if you want to track it at all for yourself. He will never know where it is again.
No, it cannot. Actually, there is no there is no drive-bay at all (standard or optical). The traditional, 2.5'' 9.5mm internal notebook drives (whether SSD or HDD) that you're thinking of are not compatible (internally) with this model.
This is what the solid state "drive" looks like inside late 2013 models:
As you can see from the iFixit photo above, the SSD is really just a stick of flash memory, connected via the PCIe bus. It's made up of 8 identical NAND flash modules (in densities of either 32, 64, or 128 GB). The 256 GB drive is pictured here. There are 8 32GB chips in total, 4 on each side. The chip density corresponds to the listed drive capacity, so 8x64GB Modules = 512 GB, etc.
Though it is possible to replace the SSD with a larger capacity one, there's currently no aftermarket upgrade available yet:
Unfortunately, the proprietary PCIe 2.0-based SSD in the "Late 2013" models is limited to a smaller "blade" option, but upgrade options no doubt are forthcoming, nevertheless.
Soon forthcoming, indeed.
Consider purchasing an external storage device, or exploring 'cloud' based storage options.
Best Answer
No
Unfortunately, There is no way to track it to an accurate location without Find My iPhone enabled. If your able to get the devices serial number your best bet would be to file a police report at your local police station.
If it was stolen back in January odds are it has already been erased and/or resold. So even if there would’ve been a way to track it, you wouldn’t be able to anymore.
Edit 1
If you enable 2 step verification, if the thief tries to log into you Apple ID on the stolen computer, it will show on your other devices an authentication code for that devices, aswell as the location of the device that is requesting permission.
So if the thief tries to log into your Apple ID and you have Two Step Verification enable, it will show you the stolen devices location.
Apple cannot track your device as it would most likely violate some privacy policy, though if you give them your consent they might be able to track your device. I would suggest you contact them and see what they can do.