Much easier...
#1). As said, the rifle is a collector and you should be extremely careful messing with it... if you care...
#2). You can "cast the bore." Cerrosafe is available from Brownells and midway and ??? Melts in boiling water. Plug the barrel with a tight patch and pore some in. Let it harden. Brief. Slide slug out and mike.
The other, cheaper option is to cast with sulphur. Garden stores have it cheap. Melt it outside. Hard to burn, but if it catches fire, it stinks. Again, plug the bore. Little WD 40 first never hurts. Pour the sulfur in. Funnel helps. Slide hardened plug out gently. Mike. If it breaks, start over.
#3). It really doesn't matter as a rule. You may find you have a .223 bore and shoots better with .224 bullets. You will not be able to tell the difference in pressure without alot more equipment than I can afford. If the case with the bullet you choose and alot more .224's are available will chamber... and you want to shoot it, go for it.
#4). This is not an extremely strong action per De Haas. If you shoot it much, it will loosen up and may lose value. I wouldn't, but it is your gun. Enjoy.