In regards to cleaning the inside surface of unlined vintage bags. I vacumn the bags out first and then give them a bath. After they are wet and pliable I often turn them inside out so I can really give the inside a good scrubbing. I often use an old toothbrush on the seams to try and get them clean. If there are any ink marks I use alcohol to remove or at least minimize it.
I have washed British Tan bags and they come out nice and look better afer the bath; but if the bag has dark stains like oil spots they often do NOT come out in the wash! Light colors, including British Tan, are harder to rehab than darker shades.
Often the worn edges and piping darken as soon as you wash the bag and apply leather conditioner and you may not need to do anything else. In cases of really bad wear, after you clean and condition and you can see the final color of the bag, you can touch up the worn areas with permanent acryllic paint.
You can buy the paints at hobby and fabric stores and they are often less than $1 a bottle. When fresh, the paints wash off easily with soap and water but after 24 hours they are permanent. Touch ups are easiest to do with black bags, but you can mix colors to get the desired shade.
Below in the first (before) pic of my poorly abused, green sheridan glenwood bag; you can see that the green color is completely worn off the corner edges. After I cleaned and conditioned her, the corners looked a little better but still not great.
I finally blended some colors to match the green and touch up the corners (see second "after" pic.) It is hard to perfectly match different shades of green (and the match didn't come out perfectly) but I think it looks better.