Not a bat maker as such but,
Of course you can make any shape you want, if everyone avoided copying other shapes then the industry would never have progressed. If you start claiming it's completely original shape created by you it's different but no issues in copying another shape
Grit of sandpaper depends really how fine a cut your tools take out, I know some bat makers use finishing planes that almost negate the need for sanding at all as they take such a fine cut and then they start with P240 (White willow Cricket) and others (probably the most common) would be around the P80-120 grit mark as their first stage of sanding. Most bat makers will go from there starting grit 120 then 240 then 320 or 400 by hand to finish it off. Some like Salix will do a lot more some may skip anything above 240 all together. A lot use white chalk based polishing compounds such as HYFIN with a double stitched mop, others use beeswax like bat wax and buff by hand with a cloth or a loose leaf wheel. What I've found is if you have a good professional set up with a good mop that has a large surface area, a compound like HYFIN is best, but by the sounds of it for you just making a few a wax product and hand polishing or small mop will work best
As for a horses shin bone, a local butchers is your best bet and you'd have to boil it to clean all excess meat off and then soak it in linseed oil, alternatively any hard wood would work as a bone, Chris King at Gray Nics uses beech as a bone as willow is so soft.
Hope this helps