1 Don't worry, and go and have a cup of tea and a biscuit. Walk round the block etc.
2 You've got an offer of advice from Johnty above: he's a real expert.
3 What would I do? It's good news that the car started once all after your modifications. Is everything else working except the starter? Ignition light coming on OK, and immobiliser not an issue?
4 What could have happened when you tidied up? It's probably a blown fuse, a faulty relay or a dislodged connection. You've worked that out already. Test all of those again, one at a time. Don't forget everything needs an earth circuit as well as a feed. This is where the multimeter will help. Can you borrow or buy one?
Jonathan