You could try one more test to see if your observed single sweep is being triggered correctly:
Remove any vertical inputs and set the A trig selector to external. Set A sweep mode to single sweep. Press and release the single sweep reset button, the green ready light should remain illuminated.
Then set the A trig selector to line and you should get the single sweep and the ready light is extinguished.
I don't have a 453 but I do have two 454s and each has had a number of failures of small signal transistors. Since you have an identical 453 available and the transistors are socketed, you can swap the likely culprits between the 'bad' and good units and check that the good one still works. I would start with Q484, Q485, Q494 and Q495, then Q575 and Q585 and least likely Q543.
My (suspect) memory tells me that someone reported 'no sweep' on a 453 and finally tracked it down to failure of C561 but I now can't find the reference to this.
Finally switches are always prone to failure after long periods of storage but I have found the older wafer switches more reliable than the newer cam operated switches.