The main issue is the willingness of UK (and France, in minor measure) to protect their imperial markets. IMHO, it is not credible to see UK entering a continental Common Market to the detriment of the agricultural produce of the Dominions (and allowing competion there from continental industry), nor can I believe Central Europe accepting tariffs on the Argentinian grain and beef to protect Australia, Canada and South Africa.
The keystone to an early Common Market in Europe is UK retreating into their imperial sphere. Pity that in 1920 France and Germany will find very difficult to agree on which day of the week it is, much less on economical policies (while in 1955 there are both a common interest - the big bad Russian bear frightens everyone - a big sponsor - Uncle sam needs to rebuild as fast as possible western Europe - and - just possibly, and put in last and least place - the Europeans starting to understand that they cannot anymore go for a suicidal war every generation.
Which means that there must be another POD before the early common market: if it cannot be after an Entente victory, it must be either after a CP victory or because WW1 did not happen.
IMO, the former is possible only if WW1 does not last very long (it ends in early 1915, with a German breakthrough on the western front) and UK stays out (no invasion of Belgium). Maybe the POD is Nicholas cancelling the mobilization order after receiving the telegram by George V, while France forces in any case the issue on the Rhine front. The Russians are late, French attacks are too ambitious, and we end up with a replay of 1870.
After a general European conference and a reasonable peace treaty, Germany and A-H start a custom union (I was reading that Kaiser Wilhelm had this idea in mind), Holland and Belgium join, Italy the same. Slowly the Balkans and the Nordic countries are pulled in. UK stays aloof, but France - after a while - is forced to apply.
In the latter scenario (no WW1 - Serbia kowtows, and pays reparation to A-H), it goes in the same way. Even easier.
I doubt that the Depression can be made less painful through the application of different economic theories (it's too early, and too much wealth is still tied up in land rents). OTOH, it is also quite likely that there is no great depression: the economic cycle goes on as before. Slowly the custom union becomes a true common market, and the safeguards are built in.
Ok, at times I am an optimist.