> black compression / mounting ring on the front
That's mostly to hold the cone. The idea that speakers could be sealed to the baffle came long after the need for heavy cardboard to glue and control the cone edge.
> just mount the bare metal speaker frame to the wooden baffle board?
That often works OK. Especially if there is any hole in the box (an "old cabinet" is likely open in back). Some seal is better, and a very-good seal is essential in small sealed hi-output systems or the crack will whistle.
> any sound difference to front mounting verses back loading?
Walk around a noisy party holding a toilet-paper tube to your ear. It strongly colors the sound. Cut a 2-inch length of 4-inch mailing tube and try: the coloration is much less but still there. The "tube" in back-mount is "only" a half inch long and 10 inches wide, but even that colors the sound. In hi-fi, they dope and damp the cone to get "no" coloration, so back-mounting is avoided.
Why do you think it "must" be front-loaded? Most guitar speakers were back-loaded. Most guitar speakers are so colored that the tube-resonance of the cut-out is lost among all the cone resonances. Many such speakers are probably developed in rear-mount, so the effect of the cutout is part of the overall result that the factory guru thinks will work well for many buyers.
Of course if the magnet hits the PT, and front-mounting clears, you do what you must.
Before you get all sticky: verify that the basket fits the hole. Oftimes a baffle cut for rear-mount is a little too small for some baskets to be front-mounted. Generally you could open-up for the basket, then later with another speaker return to rear-mount. But I hate cutting precision holes in baffles, enlarging is no easier, and if this "old cabinet" may someday be Vintage, you want to keep it factory-neat.