It is a confusing one and I think you are right - a mismatch of damping rates front to rear will transfer movement to the front and give bounciness. Perhaps the mismatch is between the KYB shocks (which are, in my experience with other cars, really firm - being a performance shock) and the Monroe shocks (which are, again at least on other cars, usually just an OE-grade replacement intended for use with standard springs. My Uno Turbo is running on Monroe Reflexes and they are not what I'd call 'stiff', which is just as well because the springs aren't stiff either and the bodyshell has all the rigidity of a cardboard box). I suggested the spring mismatch as being the problem before but I wasn't aware you were using different brands of shock absorber too... sorry about that, perhaps I didn't pay enough attention
In general I can't think of any good FWD car that has more suspension movement (when you jump on it) at the front than the back... normally the back end needs less spring rate and less damping, since there is less mass there.
To sort it out with the least work necessary...
Perhaps get two Monroe shocks for the back (fairly cheap?) to go with the standard springs at the back. That might soften up the back end again and as I think we both agree, with a flexible chassis you don't want the suspension over-hard. I don't think a soft rear end ever causes any problems with FWD (it should just follow, not set the agenda). I like the way my old 164 drives - always feels like the front end is level-riding and in control - it's a bit soft in the back end (shocks worn) and bottoms out at the back over bumps when loaded with passengers. The Spider will never have three passengers in the back, so I can't see why it has to be stiff at the back end.
And if that doesn't resolve it satisfactorily, also get two standard springs for the front end (to fit to the Monroe shocks already there). I'm guessing that standard springs are left over from lots of upgraded GTVs and worth practically nothing, and springs don't really wear out unless they rust. The reason I suggest this is because I wonder if the Monroe shocks are able to adequately control non-standard springs that are stiffer/have a higher resonant frequency. Monroe shocks are advertised as giving a well-controlled ride, but I really believe they are designed around standard springs.
If you're trying to get a firm, racy-feeling result perhaps I'm wrong to recommend Monroe shocks all round and perhaps you need some KYB shocks for the front to match the KYB shocks at the back - but, are KYBs perhaps too firm for standard springs? Ever driven a Mk2 FIAT Punto Sporting? - the bumpiest-riding standard-production car I've ever had (used to give me a headache on some roads) and it comes with KYBs as standard, feel like they're filled with engine oil...
Luckily the Punto has a rigid body to go with its stiff ride and it feels 'eager' as a result: the cornering ability is good right up until that rear torsion-beam axle gives a mighty twitch and bites back. Like I said, I prefer the back to follow the front, and not set the agenda...
Just my suggestions but it seems after what you've gone through, not much more to lose and not worth giving up. I think there should be a ready market for whatever bits you have left over... and I do think throwing away the standard springs was a mistake
Though, I did just that with my Uno Turbo as well!