Warren
Almost all the relevant case law is very old and the following assumes that you are in the UK or in a geography where old British case law will be authoritative.
Until relatively recently i.e. the Provision and Use of Work Equipment Regulations, almost all dangerous machinery in a factory setting was subject to what is traditionally described as "strict liability".
Factories Act 1961 (restating earlier legislation) "Every dangerous part of machinery shall be secured fenced".
That you can't use it if securely fenced, tough - LOTS of case law including John Summers & Sons Ltd v Frost 1955. Some touches on what is a "dangerous part of machinery".
With one exception, the Woodworking Machinery Regulations 1922, consolidated into the 1974 Regulations of the same name. Those applies a standard of doing what is "practicable" to reduce risk, carried over largely into PUWER.
"Practicable" is a much more stringent threshold than "reasonably practicable". Essentially requires what is possible "in the light of current knowledge and invention".
LOTS of case law on this as well, including Adsett v K & L Steefounders & Engineers Ltd 1953
PUWER is largely based on what was in the Factories Act 1961 before but occasionally introducing requirements to consider what is "reasonably practicable".
Then as you probably know it is about balancing risk against cost (in time, money and hassle). Then deciding whether you want to apply a disprortionality test , or a grossly disportionality test. In terms of case law this is the contrast between the judgment in Edwards v National Coal Board 1949 and Marshall v Gotham Co Ltd 1954, the latter in a higher court which chose to ignore the word "grossly".
I think that you probably answered your own question and others have served to reinforce your answer.
In terms of a report to your bosses there is no legal get out - at the very minimum what should be a self-adjusting section of guard needs to do what it is supposed to do, without workers needing to put themselves at risk trying to adjust it (and perhaps unsurprisingly being reluctant to do so).
Possibly time to sent the old chop saw to an industrial museum.