Fire extinguisher will generally hold a charge for a long time the problem is you need to flip them up side down from time to time so the powder does not compact inside. Meaning that it maybe old and if needed it may be usable but not at a 100%. Do to the powder being compacted over time.
Chimney code NFPA 211 requires SS pipe to start at the ceiling and throu the roof. It can be triple wall or double wall HT (high temp) and you have the obtion on pipe from stove to ceiling box. The best is a double wall interior pipe it has a 304 SS inner pipe and a black outer wall.
Chemfex was the best on the market untill it went out of business too bad. I do not think its a good idea if you have a chimney fire going to take a bunch of hot burning ember out of a hot stove. Thing could go down hill from there if you spill some of that in a panic. Using the Fire extinguisher on short burst is probily your best bet and shut the stove down that will start to cut off the air to the fire and choke it out.
Have it cleaned and inspected before you useing it again to insure that it did not crack or seperate any pipe further up in the chimney.