Dear Wayne.

I loved doing my project, however from a strictly finiancial point of view if what you end up with is a 45-70 double, then you can buy one for less than you will spend on the conversion.
"This project is being ta ken on due to desires beyond finacial means although safety is a important factor. "

The fact that you are asking "Can it be hardened? Should in be hardened? Is it alredy? Can this be determined?" suggests that you are not a machinist or familier with the properties of steels and their uses/testing.
If you go through my postings on my project you can see the processes that have to take place and many are not shown in detail, for example those ribs took 18 hours to manufacture. I have a fair bit of machinery and more time in workshops than I care to remember.

With the very greatest of respect and my most humble appologies if I am incorrect, from what you have written, you are not I believe in the position to complete this project without paying for some professional help.
I have about 500 hours of labour in my rifle and if I had to pay for even 20% of that time I could have bought one.

As to the substantive questions, 1900 it is proberbly low carbon steel and would have been case hardened. The tubes may well be damascus not fluid or alloy steel. It can indeed be tested a rockwell test is not destructive. If it is case hardened mild steel then no it cannot be hardened through although it can be re-cased

Would I personaly trust the lumps to make a monoblock,no.

Would I trust the action and make a new monoblock in alloy steel ( 4140 )for 45-70 at springfield pressures, yes. If that rubbish old springfields don't blow at those pressures nothing will.

Would I do it no. I would buy a Biaikal double in 45-70 file up and polish the action and barrels, re-blue and have a nice, not over the top expensive stock made to fit me by doing some overtime at work. I would invest the money I saved in ammo and shoot up a storm with the 500 hours I havent spent in the workshop.

Best regards