Yeah, there's a lot of similarities here with NATO supplying arms to a Soviet-calibred Ukraine. But at least NATO shares standards. The interwar British, French, and Americans shared little (other than the US having adopted some French artillery calibres during WW1). What the war in Ukraine tells us is that on-going ammunition supply and technical support for all those western weapons supplied to Spain would be the most important factor.
1) What do Republican forces gain?
As with modern-day Ukraine, Republican Spain would receive a confused mishmash of weapon types and calibres. The big difference from OTL would be the greatly increased numbers of weapons supplied. A western weapons supply would be most useful to the
Ejército Popular de la República if donated weapons were allocated regionally. For example:
France could supply the
Exèrcit Popular de Catalunya across its Pyrenees border as well as the EPR in Aragon and Valencia;
Britain could supply the EPR in Galicia and the
Euzko Gudarostea (Basque Army) by sea from the Bay of Biscay;
America could supply Castilla la Nueva and the remains of Andalusia from Mediterranean ports (Almería and Cartagena).
2) What do the Nationalists gain from Germany & Italy?
I'm unclear on this question. Are you asking whether Italy and Germany escalate their support for Franco in response to western shipments? If so, I'm not sure that, materially, either nation had much more to give.
Manpower might be a factor though. In OTL, Italy provided 70,000-to-75,000 fighting men (with the CTV fielding a max of 50k at one time). German numbers were about a fifth of that (albeit, with the majority being skilled technicians - aircrews,
panzer crews, etc.). So, one possible Nazi response to greater western aid might be a numerical increase in
Wehrmacht 'volunteers' in Spain.
3) What is the ripple effect with Poland, Norway, Finland, Yugoslavia and Greece?
For weapons supply during the Spanish Civil War, little if any. Poland would continue to offload obsolete kit on the Republicans for maximum profit. Greece will continue to act as supply/shipping agents for both sides.
Or do you mean what effect on those countries during WW2? If so, that gets way more complicated. Does the supply of western weapons allow the Republic to prevail? Or does it just prolong the civil war? If the latter, then the ATL has a major point of departure in March of 1939. The results might be as follows:
- Poland: A
Wehrmacht tied down in Spain means a less expansionist
Reich in the shorter term;
- Norway: As above ... no OTL dating for
Weserübung Nord (or the occupation of Denmark);
- Finland: Uncertain ... without the Molotov–Ribbentrop Pact, the USSR likely won't launch a Winter War;
- Greece: With at least 50k troops tied down in Spain, Mussolini might risk Albania but not Greece;
- Yugoslavia: Without the Greco-Italian cluster-fnck, no need for Hitler to invade Yugoslavia
4) What is the ripple effect outside of Spain? Specifically, AOI (Italian East Africa)?
Probably an improvement from an Italian perspective. In OTL, the
Regio Esercito was able to control the territories of the AOI despite 75K of its personnel cycling through the
Corpo Truppe Volontarie. Should the Spanish Civil War become extended, it is unlikely that Italy declares war on France or Britain in 1940. As a result, supply of the AOI from Italy remains a peacetime affair.
By opening the warehouses of their considerable stocks of WWI surplus as an excuse to gently kick start industry coming out of the Great Depression;
This brings up arms supply for Ukraine again. As we're seeing, emptying the stockpiles is one thing, building them back up again is quite another. That is going to be a tough sell for a peace-at-any-price British population or an isolationist American people. France - bled white in WW1 - was also very divided but I do see an angle to play there.
Beyond Depression-era economics, there is also a political angle. This would play best in a France then under a left-wing
Front populaire (FP) government. Léon Blum was originally in favour of arms shipments to the Spanish government but later deferred to British opposition. But if Baldwin wasn't so strongly opposed, Blum would have felt no threat to the Franco-British alliance.
Anyway, I'll leave that political angle here ... unless anyone wants to get into the potential of 1930s French labour politics