Difficulty selecting at its simplest could be something relating to the cable tensions - either the cable end has slipped or the outer is no longer working correctly for the bowden principle (which would obviously be a problem). Trouble shifting from odd to even gears would suggest it is the fore-aft motion of the gearlever affected, which result in rotation of the selector around the plane of the shaft, so I would start by looking at the cable that is attached to the radial connector rather than the one that goes to the bell crank that moves the mechanism vertically and see if you can move this from gear to gear by hand or if it is just as restrictive but independent of the cables.
Edited : re-reading your post again, you say the horrible noise is underneath the gearlever itself? ie, related to movement of the gearlever inside the car, not crunching at the box? If this is the case then perhaps cable bushings underneath the gear lever may have failed so the cable is itself not being moved effectively. See the dieselgeek videos on Youtube for a better view. Would need to roll up the leather skirt to the gearlever and have a look underneath in case there is any obvious debris, but then also a proper dismantling of trim around there to release the gear-lever mount would be necessary.
In a worse-case scenario, issue with the selector gubbins inside the box? When my EWQ box died it was something to do with moving across the box that had failed : I could select Reverse, 1st and 2nd quite smoothly, but there was an invisible wall preventing me moving the gearlever across to the plane of 3rd/4th and higher suggesting that something had snapped inside preventing the selector shaft from moving vertically beyond that point. Cables were absolutely fine. Luckily had a spare GPK 5-speed box at the time that was a direct swap.