Please help diagnose and solve this problem!

[Ajax Bay]

the derailleur cable might have stretched.

As @Yellow Saddle has said 'rhubarb'.

the bedding in period of cable systems

Well let's call it that, then, but not "cable stretch".

"Cable stretch” is actually a broader term used to describe the action of the WHOLE cabling system settling into place.

A broader term ("cable stretch" sounds specific, not broad imo) prone to misuse and to cause unnecessary confusion. Cables do not stretch inelastically (ie they return to their original length when any load (ie cable tension to the RD in all but the smallest sprocket) is removed).
"Cable outers settling in" ??