The door lock issue could be the aftermarket security system as the owner suspects, but it could also be a problem with the driver's power door lock actuator. The actuator in the left front door is the master unit and can cause some weird issues if it goes bad.

The smoking issue is due to worn valve stem seals and is very common on the Monteros. It is a pretty significant repair on that engine, even if you can do it yourself, but if the smoke doesn't bother you it can be driven for many miles as it is. Like any postponed repair it can affect other things if ignored - damage to your catalytic converter is possible, as well as potentially failing emissions if it gets bad enough.

The wiggle in the steering could be any number of worn suspension or steering components but the idler arm is the most likely culprit, especially with the relatively low mileage of that vehicle.

The rear door problem is due to the plastic exterior door handle breaking - I haven't dealt with it personally but others here have. The search function is your friend for that info.

You will want to check the variable intake butterfly valve shaft for looseness - another common problem with that engine. The shaft bushings wear and allow the shaft to vibrate which can eventually loosen the securing screws for the butterfly plates allowing your engine to swallow a nice little chunk of itself. Replacing the lower intake manifold with the butterfly valve assembly is easy if you're planning to have the valve stem seals done. Again, the search function should yield many posts about this issue and how to diagnose & repair it.