Poland wouldn't attack East Prussia so quickly. From Polish POV a conflict between two German states is a good thing, not to mention Poles have no reason to like Red Germany. So as long as East Prussia sits quietly, Poles let it be.
Red Germany can indeed hit Poles in the back when they fight Red Russia, but it would have meant Entente intervention. France and Britain are exhausted, but they can still beat Germany. Honestly, I think that the very success of German revolution might trigger Entente intervention.
But let's assume a scenario like this: German civil war takes a little longer. Nor France, nor Britain are willing to interfere, as long as Germans are killing each other. Besides, with SPD the Reds they might be seen as not so bad. Poles defeat Red Russia as IOTL, since Germans are too busy to interfere. Eventually, in 1920 or 1921, White (Black?) German forces are defeated and withdraw to East Prussia - mostly be sea, with Poles magnanimously granting the rest passage across the Corridor (unarmed and under guard). Why? Because, as I mentioned before, weakened Germany is in Polish interests.
Now, we have an interesting situation. A socialist Germany (not communist - I still think it would have triggered an intervention); and a reactionary Germany in East Prussia. Between them - the Polish Corridor.
What is economical situation in East Prussia? Not good. IIRC East Prussia was mostly a rural area, with agriculture being a basis of its economy. The Prussians have no natural resources and only little industry.
Politically, they are also in poor position. Very few people in Europe has any sympathy to successors of the government which started (in their opinion) the Great War. Some western goverments or right wing movements might see thenm as anti-Reds, but they are simply too weak to be treated really seriously. The facts are simple - the main Germany is red, and if you want to do any business or make any political arrangements with Germany, you have to talk to the Reds.
Militarily, Prussians are also weak. In the beginning they might have relatively strong army, but without industry they can not mantain it. Their ability to produce new equipment, ammo etc. is minimal. They also lack soldiers. Before you say they can expand to Poland or Lithuania - no, they can not. The moment they try to do that, Poles will fight them with everything they have. And they will have superior numbers and will to fight an attrition war which Prussia can not win. Should Prussia invade Lithuania, Poles will immediately intervene to protect their "Lithuanian brothers", whether those brothers wish it or not. Why? Because strong Prussia is not in Polish interest. And an intervention or "liberation" of Lithuania might give Poland an opportunity to install pro-Polish government in Kaunas.
In short, East Prussia will be weak and disrespected. To survive and gain any strength they would need a protector - just like Taiwan had USA. Problem is I do not see any country willing to be such a protector. Soviet Russia? No way - they will be more friendly to Red Germany. Britain? I do not think so. Prussians simply can not give Britain anything in return. France? NO WAY IN HELL. Poland? Poland is too weak to be a significant protector and has no warm feeling towards Prussia at all.
IMO the Prussia soon will become a kind of reservation for reactionary Germans. Surviving only because Poles refuse to allow Red Germans to pass across the Corridor to finish the job. To be honest, Poles would be very gleeful about reactionary Prussians being more or less on Polish mercy.
A civil war in Germany (especially a longer one) might also cause some border changes. With Red Germans and Black Germans fighting each other relatively more peaceful Poland might seem to Silesians and Masurians more attractive, especially after 1920. Poles might actually win some plebiscites (if they happen at all - it is hard to make a plebiscite in a country torn by a civil war) and gain more territory in Upper Silesia. With East Prussia being sepparated from civil war I think plebiscite there would end as IOTL. Danzig Germans decide they are quite happy being a Free City- Danzig businessmen might not be happy with becoming a part of Red Germany, and being part of poor and reactionary Prussia is also not so tempting.