With a minimum order of 600' and a min. diameter of 5/8", that is definitely NOT the proper rope for a 9'H x 22'W traveller.
If they "just used cotton rope" before and it needs replacing, would buying a better quality of rope be more cost-effective? Possibly
Iweiss:
Wire Center Rope, Cotton, 1/2" diam. White, Galvanized Wire Center,
WLL = 236 lbs
Used on tracks when moving scenery or maintaining spikes.
$530.00 / Spool (1000' spools)
$0.56 / Foot
(WC500CT)
Spray-on fire retardant should not affect the life or usability of any cotton rope. Measure the floor block, live end blocks, and dead end block; and use whatever diameter they want. Should only need about 65' of operating line. Wire-core cotton rope should be long lasting, and will not stretch over time.