I will consider adding this later

We will consider adding support in 2-3 months.
Generally i like the stylus, but i dont like it uses “disc” tip, but i suppose it is the only good
Sution for battery-free stylus

More undo steps - it is only problem with memory usage. Each undo step consumes a memory. Of cours some undo steps eat for example only 1MB (like painting small stroke), other can take 100MB (resize image with a lot of layers).
When all ram memory is used, app starts swapping some data to disk.
All in all, I will consider increasing it.

Autosave - we  both your requests already done. They will be available in version 1.1 in about 1 month.

Some email apps are compressing images.
To make sure image is delivered in original resolution there are 2 options:
1. Export to PNG and zip it (you need app from App Store to do it)
2. Export in PSD format (flatten document before exporting so file size will be much smaller)

You can also drag a preview from to top to thumbnails are and add/overwrite. You have to do it after any change to save.
It will be improved in version 1.1

Could you give us few examples of these buttons?

Watercolors

Cursor together with drawing offset will be added in version 1.1