How would I be able to emulate this snake's scale pattern that maintains shape throughout the curves?
I can go in and hand draw the face afterwards but as of now I'm pretty stuck on a solution without trying to draw it from scratch.
I can think of one way you could do it (or rather, fake the 3d look) in Illustrator. You could create a Pattern Brush.
Then you could apply the brush, to a path, and select a Variable Width profile.
Here's a rough example showing the basic concept. The Pattern Brush itself is shown top left.
Then you could do this over the snake illustration, and use a copy of the snake's outline as a clipping path on the pattern. Obviously it will require quite a bit of tweaking to get it right.
The Variable Width profile is useful because you can also use the Width Tool on it, so you can control the width of the pattern if necessary. For example around the head and neck area:
Wrap around=you need 3D model. In 2D you can make an overlay which looks like it's wrapped around.
You already have the big pattern, I see. The scale pattern could have been made at the same time.
But let's talk about the missing pattern. There are good snake drawing tutorials like this:
https://design.tutsplus.com/tutorials/how-to-draw-animals-snakes-and-their-patterns--cms-21338 Obviously you have found some of them.
You can create a piece of the missing pattern and make a pattern brush. Then you draw with it the wanted curve and use blending modes and reduced opacity to mix it to the underlying texture.
With the stroke width tool you affect locally to the diameter. A simple example (the pattern is crap, but it is only for principle)
Your target pattern seems to be something like this, but more complex. You have got a good explanation of it in another answer. This is a simple mesh:
Draw a symmetric S which has equally long horizontal handles at the ends. It can be made exactly by joining a half of it and a 180 degrees rotated copy. Set the horizontal width of the shape = 100 mm. You will need exact numbers to make all fit. You can scale later.
Make a horizontally flipped copy and arrange them horizontally and vertically.
Make horizontally shifted copies. Use Object > Transform > Move > Copy, set the distance = 10 mm. Repeat with Ctrl+D.
Draw 2 vertical division lines. Place them with the direct selection tool to exact endpoints of the S-curves
At this point you must convert your S-curves to outlines, because dividing strokes is difficult. Select all except the division lines and use Object > Path > Outline Strokes.
Your pattern is ready to be dragged into the brushes collection. Define it to be a pattern brush. Here's a piece drawn with it:
You cannot change the wire thickness of the mesh. It must be right when you draw the symmetric S. Test carefully before the needed stroke width.
The symmetric S presents helix curves around a circular profile. It's wrong if you expect more square-like cross section. You should add rounded angles. Beware: You will meet easily geometrical problems because it's not the same from different directions. I suggest use a round profile for scales and fake the squareness with bigger parts.
You can get closer your model pattern if you fill some diamonds with the shape builder before you make the brush. To get the centerline of the patten flow like in your model from side to side, you must make a wider scale pattern than needed and mask it with a clipping mask on the snake.
Your model snake has consistent lights and shadows. You must make a separate overlay for them, it cannot be included to the pattern brush. I can see you already have thought it to some degree.