The easiest DIY solution (that I think would look decent) would be to get some (gel) wood stain in the closest color you can find to touch up the scratches then remove the handles and do two coats of (satin finish) polyshades in a color slightly darker than the doors are now (probably bombay mahogany).

I squirted the gel in all of the grout lines.

Scrub the walls first, to remove as much nicotine as possible.

Additionally, the residue that's left behind can cause some paints to literally slide off the wall, much like you see with heavy nicotine staining, but much, much worse.  

The ozone generator sounds great, but in our experience with turning rentals, we still have to do a lot of scrubbing before we paint due to the tar/nicotine.

I'd go with Combat Gel to get rid of roachs.

The tips tend to be the darkest and what soak up the tar/nicotine in the air.

How did you get rid of the nicotine on the walls?

If so, how labor intensive is it to remove the smell and nicotine from the house (i.e. walls, air ducts etc), that is, assuming it's possible?

Secure bank financing.House, at the very least needs a serious paint/nicotine removal job, new carpet/flooring throughout, some junk removal in garages, new refrigerator.