This is going to be where to start from problem as you have done quite a lot of wiring changes and you started from a point of things not working.
OK so from your post you wired the socket directly to battery, via an inline fuse, and not via ignition, and the fuse is intact so initialy I would discount that as directly causing a poroblem, if it had shorted the fuse would blow, you cant connect it the "wrong" way round, (in terms of causing a problem at the moment)
If where you were fitting socket or drilling you could have damaged or dislodged a wire than that could have been the problem or it could have been total coincidence, but unless you had a history of similar problem I don't like coincidence,
Have you marked the wires on loom and old immobiliser in such a way that it can be wired back in, If it was me I would try to return to where I was prior to problem and start from there.
other than that its just going to be a session with the multi meter belling out each circuit involved .
Short of a plate buckling a battery cannot go instantly flat so measure voltage with it disconnected then connect terminals, measure Voltage, turn on ignition and measure voltage, they should all be pretty much the same reading, voltage should only drop when cranking, if something is a direct short across battery it will burn out or get very hot very quickly.
Tim