Is the oil pouring on the ground as you add oil? If yes, the oil drain plug could be missing, or installed loosely; the oil pan could be damaged, the oil pan gasket could be broken or missing, the oil dipstick pipe could be loose where it meets the engine. Are you a first time car owner? Have you overfilled the engine with oil perhaps?
removing a battery terminal can often corrupt the CPU program in the ECM
first try using the scan tool to reset the ECM
check for cam sensor wires /loose connections from moving the harness around to remove the plugs
Have you had it hooked up to a scan tool ? check sensor data ! Your vehicle has no cold start valve . Black smoke out the back indicates a rich condition , holding gas pedal to the floor causes the engine computer to go into WOT mode shutting off the injectors . Suggest you do injector leak down test , leaking fuel injectors ! Ever replace fuel injectors ?
The M8 bolt that secures it is easy to remove, but the sensor is often rusted in, especially on the rear. If so, you will have to remove the rotor and hub. The ABS sensor coils region is about 13mm diameter, the sensor magnet tip is about 6mm. Find a LONG 1/4" shaft socket with outside dimensions a bit less than 13mm. This will fit over the magnet tip. The base of the socket sits on the coil. GENTLY tap the socket, and if you are lucky, the ABS sensor will slide out without destroying itself. Be prepared to replace the expensive sensors if you damage them. Parts suppliers are much cheaper than dealers.
It almost sounds as if you had some sort of security system in the car that disables the ignition and prevents the car from starting. How do you know that you have power to the starter? Does the engine actually turn over? If that is the case, you might have a bad wire connection from the starter to the main power supply.
If your car has a cable speedometer it needs to be removed from the transmission to install it on the cluster first. Lift up the rubber boot that covers the end attached to the transmission. Now find and remove the retainer clip, take a pic or sketch how it looks installed before removal. Some versions of the cable use a hex threaded end and will unscrew with a wrench.Now you can pull the cable out of the transmission do no use excessive force it will just pop out. With the slack you can now easily attach the cluster and the electrical plugs.
Noise lifters on a 2.5 engine of the first generation are an unusuak problem; these engines are known to have "piston slap", which can sound fairly similar to noisy lifters. I owned a Forester with those symptoms and drove the car for more than 3 years without doing anything about it. The only real cure would be an engine rebiuld, which is , in most cases, not worth the effort, especially since most of the "stricken" cars continue to work without any problems for a long time.