In your preferences, go to "Startup Disk". Highlight the drive that you'd like to boot from*, and then restart your machine. *note: the drive that you're booting from must have a valid OS installed on it. If you want to clone the one from your old drive, I recommend: http://www.shirt-pocket.com/SuperDuper/SuperDuperDescript...
iFixit does sell logic boards. Find them here: iPhone 5s Logic Board They are, however, carrier locked -- so would not be useful in the UK unless you paid the carrier to unlock them here. If the phone that the logic board was removed from is not under contract, the carriers are required to unlock the board by law. You'd still need to boot the phone to get the serial number in order to ask the carrier to unlock it. Be careful though -- International phones may require different attennaes than you will find on a stock USA GSM logic board. You may not get proper reception.
Light leakage like this is a sign of a defective screen. Take it back to where you got it and complain -- it's unlikely that dropping it caused this -- if you didn't drop it hard enough to shatter the screen, then this light leak shouldn't be happening.
This is called a "boot loop", and has a number of causes. Usually this can be fixed by doing a "hard reset": Press and hold both the Sleep/Wake and Home buttons for at least 10 seconds, until you see the Apple logo, release and it should boot normally.
A water damaged phone will generally need complete disassembly to clean corrosion from the circuit board. Simply drying it out will not remove the corrosion, and the corrosion, once started will continue to get worse. Unfortunately, replacement parts for the Blu phones are pretty scarce. And water damage will require at least an hour of work plus possible replacement parts. You have to balance out the cost of this vs. buying a new phone. Blu phones are generally cheaper to replace than to fix. Unfortunately, this means more toxic e-waste in landfills.