I think what you're asking to do is simple using Expressions.
If you alt-click on the stop watch of any keyframe-able control you can write an expression to connect that control to another control.
So if you want to control the end of ramp's x position using the start of ramp's x position in your example, it would be:
x = 4480-(effect("Gradient Ramp&...
Press the triangle marked "Add" next to the Contents of your shape layer.
Add a "trim path"
Add a keyframe to the "End" parameter of 0%.
Move along your timeline.
Add another keyframe of 100%.
Your path will draw on.
You probably don't want to fill it until it is drawn.
Assuming that it's not changing perspective or moving around too much, I would use a tracker on the tattoo and a clone stamp from another area of skin. More elaborate removal could be done with Mocha, or using a mesh warping tracker like Lockdown, which has a demo of tattoo removal in their showreel at 0m24s:
Apologies if I've misunderstood, but you can just click on the Add triangle, and add a fill or a stroke to your shape:
The modifiers in the "Add" menu are hierarchical - so you have to add them below the path's hierarchical level.
You'll have to make as many duplicates of the layer as there are colours. Apply the appropriate key to each copy, and place the layer you want it to reveal beneath it. On the revealed layer use a track matte, and set it to "Alpha Inverted…". This will hide the keyed layer and use its alpha channel (transparency) to reveal the layer underneath it, ...