You need to post much larger photos. As to your questions:

1. The top safety is part of the design to his 1880 single shot, each and every one had this feature standard.

2. It is possible that Fraser would rechamber, though more likely he would just rebarrel. I don’t think this would be the case either way on your gun.

3. There is a BP 360 express its a 2-3/4” round made about 1873, made with many different variations from 71 to 215 grains. The 2-1/4” was the most common however there was also a legnth of 2-7/16” loaded as a favorite of Alex Henry who was Fraser’s brother in guns. You need to make a casting of the chamber to really understand what you have. If this is a 400/360 it stands to reason its a Fraser 400/360.

4. Fraser did not mark his barrels on the top with charge or bullet weight, part of that information would be under the forend and would be stamped in.

5. The 500yd sights were standard on guns larger than 350 bore, and does not mean the gun was sighted for 500yds.

6. The proofmarks would be the same for the era, not per calibre. Unless marked as such by charge such as nitro or BP.