a. Resplicing the chain rope join is
maintenance and should be done every 2-5 years, depending on wear and corrosion of the chain. You trim a few links from the chain, trim 5-10 feet from the rope, and re-splice. Used rodes are not difficult to respice, but riggers never want to fool with old stuff. Old double braid is a bugger to splice, and old rope in general can carry liability. It's just not worth it to them for the meager charge. This is something every sailor should learn to do. It should NOT wait 10 years, because there is almost certainly corrosion under the splice. The easy work of 15 minutes. (note the link I am splicing to is clean because I removed several)
b. Samson has a good guide to rope.
Samson Rope Guide. Also Marlow.
Marlow rope guide. Age is not mentioned by either. See the pattern? Serch NER and other major manufacurers and you will see the same. Years alone are not an important factor. It's not hemp. Just like age alone means nothing for fiberglass, as it does for wood. Cancord likely has no basis for their statement, other than the general observation that older ropes may have been treated poorly at some point. The advise is not uncommon, but has no scientific or engineering basis.
c. I can't imagine an insurance company could require purchase records for rope. Silly non-sense that could be challenged, and no accepted meaning for interpreting age anyway. What would the number mean? Nothing. If they needed to prove the rope was unsound they could test a section, but it was almost certainly made unsound by something other than years. The most important thing is chafe... which comes back to the need for maintenance.
But, after 10 years, your chain is probably corroding and you want a new rode. Go for it. The rope portion will make good docklines. Chain can be used for kellets and tie-offs. My last rode got used up in such manner.