Robert: That certainly is a beautiful looking rifle but the pictures you posted of the bore were, as you stated, not for the faint of heart. It appears the black powder corrosion has completely ruined the rifling. I know you had mentioned lining the barrel and since you intend to keep the rifle in its original external condition I think this would be the best way to go. If you consider a re-barrel, you might wish to contact the Montana Rifleman and ask them if they work on or restore original 1873s to the new condition.
I don't know what condition the barrel is in other than the completely corroded bore and do not know if what remains of the barrel is either strong enough or integral enough to withstand the lining process without developing cracks or becoming otherwise fatigued. HTH.