Not a mage so I’ll skip the first, but for the second, you’ll want to have a good read of the armour rules here:
https://www.profounddecisions.co.uk/empire-wiki/Weapons_%26_armour#Armour
Specific points there which seem relevant are:
Any character can wear armour, but magicians cannot cast spells or perform rituals while wearing armour.
To count as armour, the phys-rep must be a phys-rep of a suit of armour. A leather coat or biker-jacket does not count as armour and is not appropriate costume for the Empire setting. Leather armour may count as light or medium armour depending on the thickness; adding studs or rivets does not make it count as heavy armour.’
So if it’s mage armour, you don’t get the extra hits. If it’s regular armour, you can’t cast while wearing it.
And under Medium armour:
Thick leather armour (the majority of the armour is more than 3mm thick) or any armour made from a light weight material such as polyurethane (e.g. Norton Armour), aluminium, or mixed neoprene and steel provides the wearer with 3 extra hits. Light mail, such as modern ring mesh (butcher’s mail) or mail with an open weave (sometimes called “ring mail”), counts as medium armour.
So if it’s actual bone or plastic/resin/latex to give the effect of bone, you are looking at Medium armour at best. If it’s actual bone, you will want to be very sure it can stand up to being hit and more importantly, being fallen on by you and/or heavier people than you in full plate! If it splinters or breaks it will be a safety hazard and Weapons Check will not allow it to be used on the field.
As it’s an unusual material if you are wanting to use it as regular armour I would email PD for a ref call, explaining what you have/are planning to use and asking for a call either way. They may well want to hold their answer until one of the Weapons Checkers have a chance to inspect the armour in person. For mage armour you can use any material that is safe, so you just need to be sure it’s robust enough.