Hello Andi and welcome to the forum.
I can start to answer some of your questions as we own Super Seal Mk 2 #131 1987 which shares some of the construction..Your photos are a great help.
The keel has suffered from a serious lack of maintenance over the years -perhaps the boat was not lifted out at regular intervals because of its location.,so mounting problems were not addressed.
1 The acetal slides on the keel appear to be out of parallel - one side appears to have become loose on its bolts at some point ,even if they are tight now. I had the same issue and it must be a common problem.The acetal strips will tend to slide on the grp surface and get displaced .The fixing holes can then become oversize and the fit sloppy.I went so far as to add thinner FG strips inside the slides so that they could not move inwards.The slides themselves need to be proud .
2 The assymetry of the opening seems very unusual because the 'widenings/cavities' have been cut back to the cast iron ballast plate .It is possible that pockets were attempted here in order to fit an ad hoc non-standard roller - I cannot see any other explanation. This could have been done after the original nylon blocks became overworn.I believe that rollers are only standard on a Parker 275 which has a substantially heavy keel of different design.The grp hull originally overhangs the cast iron to give the precise opening.
3 The keel is originally provided with the nylon (later acetal) blocks shaped to the leading and trailing edge to give fine control fore and aft as the keel is lowered through the hull aperture.These must be sacrificial to some extent as they keep the keel away from the cast iron and the grp.They are fixed with large machine screws into the cast iron,which has recessed pockets to take them.
When I find it I can send you a photo of the trailing edge block which I have replaced from our hull-because it's groove had very nearly worn back sufficiently to allow contact of the keel with the cast iron.
4 It seems that the trailing edge block on your hull became so worn that the keel was allowed to wear heavily into the box itself ,and the obvious wear there surely took many years to form.Surely the trailng edge must also have suffered .
These things together would certainly have made it sometimes impossible to shift the keel.Hard to see if the final hull slot is itself assymetric ,as well as the rough side cavities and whether the actual box is dangerously compromised and needs repair .
The fore and aft blocks are easy enough to form cut from acetal and to replace provided that the threads in the cast iron are still usable .
But to reform the final opening my initial thought would be of a thick surface plate that gives wide spread beyond the hole for strength ,possibly in fibreglass and bonded to the hull.Not the most desirable but easier than trying to structurally extend the existing fibreglass 'lips' which actually take the side thrust of the keel.
Best wishes ,Peter