However, since Leon just let this happen, my point was that this suggests (not conclusively, I concede) that Leon was still low on manpower. Also, when Leon did try to reverse the Navarrese takeover of Castile, about four years later, it lost the war.
1) It's less a case of manpower there than political influence. A ruler's minority is always a bad pass, and it happened that Sanche III managed to impose his authority during the regency, mostly trough dynastical ties with Pampelune (Castille and Pampelune sharing a dynastical union at several levels).
Remember that Alfonso had to deal with Almanzor campaigns consequences : less desertification (even if taking back control of 2/3 of the realm wasn't something easy) than civil war and disunion.
2) Giving the
probable (nothing is certain, of course) balance of power, and giving Navarre situation, it's less highlighting an objective lack of manpower to Leon than Navarre being on par with Leon on this.
If I'm reading your linked map correctly, Navarre itself was around the size of modern Luxemburg when it took over Castile and shortly thereafter defeated Leon, which puts the matter in perspective.
Well, it's not the size that matters.

For example, Luxembourg is still quite small compared to Spain, it doesn't make it arguably wealthier.
Furthermore, you discard the net of influence that made Navarre quite strong at this point : Navarre+Castille for starters, were arguably one block (and that increased the size of the kingdom of Pampelune quite importantly), and influence if not hegemony on Western Pyrénées (remember that Gascogne up to 1032 was in Sanche III's "sphere of influence" if not under his suzerainty, and that only double the size of Navarre+Castille+Aragon+Sobrarbre)
It did helped that a good part of Leon (roughly the third) was something close to a no man's land, while Navarre was arguably more safe from Umayyad raids. (not meaning Almanzor plundered up to the ground Leon, but that regular raiding during 300 years in the southern part of Leon had some consequences).
This map could give you a clearer view on the situation prior Sanche III campaigns in Leon.
A Luxemburg, maybe, but a Luxemburg on steroïds.
I understand your argument, but there's a second butterfly at work. The thread starter wanted the Normans to go to Spain instead of South Italy. If you work from the assumptions (i) that the Normans aren't invited into Italy, but (ii) are actively recruited to northern Spain, you could see larger numbers down there, including the dynamic early Hautevilles.
You misunderstood me : I was making an OTL comparison for numbers, as in TTL Norman presence would be more reduced than OTL numbers involved in Italy.
1) You had a Norman presence in South Italy, invited or not. At this point, I think that at least a reduced maintained presence is likely, in Sicily for exemple.
Thinking about it, I don't really see how you can butterfly it easily with a Spanish PoD : Drengot presence in Italy was already something at this point. (Altough, I agree, it could maintain its presence as Byzantine mercenary, rather than see Normans servicing Italo-Lombard nobility)
That said, I concede that a more reduced presence in Italy would benefit Spain, and vice-versa. I just don't think you'd have more than some handful hundreds (something quite enough to establish fiefs, of course)
2) I don't see it being "actively recruited". Spain was less known by Normans, less politically divided (you can really more easily create your own fief or take over one when you're opposed to pocket duchies rather than two relativly important entities) and less rich (at least, if you consider North Spain)
Furthermore, dynasties as Hauteville are likely to be if not outright butterflied, at least greatly reduced. When his sons embarked to Italy, Hauteville was but a minor noble.
Their fortunes were directly issued from fortunes of war, and these aren't definitely written down.