Does the Allez have a functional wheelset now? And what about the current group? Yes, (I'm no expert) but I think there will be issues modifying a wheelset to fit 126mm and hoping it can be reused for 130mm+ later on.
I can see making a few mod's if you really like the feel of the frameset and want to keep it going a little longer. But some of the things you are discussing are just throwing away money that should be tucked away towards a complete new bike (IMO). Cutting threads into a new carbon fork? Modifying a new wheelset to fit 126? All for a 25 year old bonded carbon/alloy frame with a broken integrated seat clamp? I certainly wish you the best - but I'd be making some different choices personally.
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"The older you do get, the more rules they're gonna try to get you to follow. You just gotta keep livin', man, L-I-V-I-N." - Wooderson
'14 carbon Synapse - '12 CAAD 10 5 - '99 Gary Fisher Big Sur