I've just read this thread from the start and no-one has said anything about the cost for Ji when the cylinder is due for testing (10 years from the last test date in the case of Argon). When I worked at BOC (and ran the cylinder test shop at Rocklea, Brisbane) we used to do testing of privately owned cylinders (RAAF, army, Telstra etc) and it was about one hundred dollars per cylinder (in the 1980s). God knows how much it would be nowadays, and no gas company would fill Ji's Argon cylinder if it was overdue for test.
About the heat indicator tags, they were developed by BOC during my time there and it was because there was no way to tell if an aluminium cylinder had been heated slightly above the temperature that causes softening of the metal over a long time scale, and some aluminium cylinders failed during filling due to being exposed to temperatures of about 120 degrees for a long time (the heating was while they were with customers). The previously used heat indicators (test date ring, stickers, valve handwheel and paint) were not affected until the cylinder is heated above the point of softening so we developed those saggy tags that droop if the temperature goes over about 100 degrees C.
It wasn't a legal requirement when I was there but may have been added the Australian Standards since then. We mainly wanted to protect our cylinder fillers from exploding cylinders.
Aluminium gas cylinders are very carefully age-hardened in a salt bath when new and can be just as easily age-softened by customers.
Ji's argon cylinder is not BOC and not aluminium so does not have or need a droopy heat indicator tag.
As far as welding steel with oxy-LPG goes, even with the right tips, it is very hard to control the weld because the temperature throughout the length of the flame is almost constant. Oxy acetylene is so good because the steep temperature gradient moving away from the tip of the bright inner flame makes it easy to control the temperature of the weld pool.
For flame cutting, oxy-LPG is easily the most economical combination and works great.