Scarcow, huge improvement – great job on the area transitions and well done for taking Simon and Bogie’s advice regarding your tiles on the lower-right side of the map. Regarding the three points you raise:
1) your map already has nice tonal variety so I don’t think you even need to do a lighting layer.
2) Making objects stand out – yes to a point shadows can help but ultimately the key problem is trying to cram too many different fiddly little objects of different styles and clashing colours into a map rarely works. In my opinion it is better to select fewer, stylistically-consistent objects and make them a little larger. Stuff like rugs, furs, piles of clothing, etc. can be hard to identify and just clutter the map without adding anything. These sort of things are never going to stand out the way you want them to. The way to make light sources stand out is, of course, to create a light/dark layer. Specific to your map, I would advise standardising bed styles, wall-sconce style (I really like the ones in the tombs), wood style (see your clashing bench and table), brazier style and sarcophagus style. Don’t forget with GIMP you can easily recolour, desaturate, rotate and brighten/darken each individual object. Well that's all just what I think anyway - obviously some people love all lots of detail and variety so feel free to ignore.
3) Cave entrance to the south? Why don’t you extend the cave tunnel to the southern edge of the map – that should do it and is simple and effective.
4) Yes your circular steps ovelay in the bottom-right is not working. I would delete that and start again. To make rough stone steps leading into darkness you should do them by hand – it is quite easy. Step One, you make three-four steps. Draw the step outline, then draw a fading shadow below each one. Step Two, select the entire area of the steps/tunnel end and use the gradiant tool (set to transparent>black) to fade it out to black as you go down the stairs, so your lowest step is virtually impossible to see.