Illustrator – paint/draw inside the drawing created from brush strokes, not shapes

I have been struggling with this for last few days and decided to get expert help here. I have drawing like below (original art is more complex) which was created using brush strokes (no built-in shape objects). I am trying to paint/draw inside using brush strokes just like “draw inside” feature of Illustrator. As it works for closed shapes so I thought making sure all lines are connected will suffice, but it didn’t.

Use clipping mask to fill transparent shape in photoshop

I have a rectangle in photoshop that is transparent with a black outline, and I’d like to make a frame-by-frame animation of the rectangle filling with red. I know there are a few ways to do this but right now I’m doing it by creating a clipping mask and slowly bringing the clipping mask upwards, saving every time, and then playing the images back quickly.

How to make sure the clipping in a .svg created with Inkscape are imported well in Illustrator

I received a .svg design made with Inkscape and I need to convert it to .ai to send it to a printer. But when opened with Illustrator, lots of features were changed.
Most notably, some of the masks (“clipping” as Inkscape calls them) are not taken into account, (objects that should be cropped were suddenly not) although strangely some of the objects were cropped properly.
I also spotted a few issues with the texts, too, but this is easily fixed.