I would disagree in some things.  I don't believe it's any sin to remove dirt, grime, and copper/lead fouling.  In the bore of a firearm, electrolysis is by FAR the preferred method of doing so.  There are fairly benign methods and chemicals for removing external, active rust on firearms, and doing so can be tricky.

I have not experimented with cotton or wool fabrics, but it is possible that a soaking in alcohol might help in reducing mold/fungus, BUT the alcohol might remove some of the dye in the fabric.  Proceed with caution.  Obviously bleach, a sure-fire mold/fungus killer is contra-indicated.  Some exposure to direct sunlight will kill mold/fungus, although at the risk of fading.  Sometimes a good wash in cold water, and thorough drying in the sun will do the trick.  Treat any leather with Pecard's before washing the cotton, in order to protect the leather.

In regards to leather, the ONLY thing I use on good leather, particularly antique/fragile leather, is Pecard's leather dressing.  The folks at the Army War College Museum require it for their museum displays, and that is good enough for me.  None of the new leather items I have treated with Pecard's have shown the slightest age-induced deterioration over decades.  Wear and tear is another thing, and apart from the care applied to the leather.

If your leather is fragile, as in cracked and very dry, Pecard's will stabilize it at the point when it is applied.  I wouldn't use antique/fragile leather under any conditions, except for display only.  Find/make a substitute if you want to actually use it.

Interesting. I know of people who have carefully restored M1 Garand rifles to as-issued condition, and sold them off, at a premium price all the while saying the rifle is a restored rifle, not  original, or anything else.

FWIW, in a hundred years, with the loss of documentation/provenance, many commonplace, restored rifles will be worth an astounding amount.  Assuming we are allowed to own them.