Hello there,
yeah the dreaded weak pedal box this happens to the MK3 Polo's also.
Quite often the clutch cable wears causing excessive drag which can result in a stiffer/juddery feel when operating clutch, plus if the clutch unit itself is warn can cause the pedal box to fail, as the welding is small and not much of it which is why any extra pressure on clutch pedal finishes off the pedal box unfortunately.
But yes the cable operated clutch on the MK2 SEAT do tend to feel a little heavy, like I say my MK3 Polo always felt a little heavy, I think they use the same gearbox the Polo and
Ibiza.
To be honest all cable operated clutch feel like this, I've never found a nice light feeling cable clutch pedal, I've driven Peugeots, Ford's, Vauxhall's and VW's with cable clutch.
Admittedly if the clutch pedal is stupidly heavy or has sunk slightly, then clutch unit could possibly be close to wearing out.
On my old MK3 Polo that I used to have that had a clutch cable, always did feel slightly heavy plus they normally start to creak when the cable is getting close to snapping, well that's what happened on mine.
I think the manual adjustment clutch cable is fitted to 1.4 possibly 1.6 and the much smaller engines, whereas the auto adjustment cable I think is fitted to all or most engines above 1.6, plus they're possibly not interchangable as they're normally different lengths, whether that is just down to the ratchet/spring mechanism in the auto adjustment cable, I'm not sure.
I think the manual adjustment clutch cable length is 1655mm
The length of the auto adjustment clutch cable is I think 1852mm
When you get a chance to replace your clutch cable, before you attach it to clutch pedal
I'd definitely make sure/check that the pedal does move freely. With the new clutch cable fitted it should feel slightly better, if not then chances are your clutch unit could well be the issue. If you have to apply extra pressure when pushing down clutch pedal the last 1/2" to 1" of it's full travel, then this is a clear sign the clutch is going plus it'll feel really heavy.
Also the answer to your other question is yes,
a new auto adjustment clutch cable will self adjust when a new clutch is fitted, unless the cable mechanism is faulty.
Hope this helps
Si