Thank you for your help!
Happy to help. Sorry the information isn't what you want it to be.
As I understand it, no one is "entitled" to American Citizenship unless they are born in the United States or are naturalized as an American citizen. He very easily could have gone to court to get his citizenship affirmed and had it denied, as there is no "entitlement" of a foreigner to American nationality regardless of how long they are resident in the USA.
You're entirely right there. However, even if your grandfather wasn't entitled to US citizenship in 1920, he lost his Polish citizenship the moment he took US citizenship prior to 1962 (and so had no Polish citizenship to pass on to your father). And even if your grandfather didn't take US citizenship prior to 1962, your father lost his Polish citizenship the moment he took US citizenship prior to 1962 (and so had no Polish citizenship to pass on to you).
Have a look at the 1920 act on Polish citizenship:
Art. 11. Loss of citizenship happens by:
1) obtaining another country's citizenship;
Most people seem to say that since they were Polish citizens, they were obligated to serve in the Polish military, and say that if they didn't receive permission not to, then they weren't permitted to lose Polish citizenship.
They weren't permitted to lose Polish citizenship because they didn't obtain permission not to serve? Are you sure you've said what you wanted to say there?
no one served in the American military (except during the time except in WWII for allied forces)
Serving in any armed forces other Polish at any time between 1920 and 1951 without the permission of the Polish government resulted in loss of citizenship.
Have a look at the 1920 act on Polish citizenship:
Art. 11. Loss of citizenship happens by:
2) taking a public office or entering the service in a foreign country' army without Polish government's permission.
naturalization papers
If anybody naturalised as a US citizen before 1951, they lost their Polish citizenship, no exceptions; see the article above. If anybody naturalised as a US citizen between 1951 and 1962, they lost their Polish citizenship unless they had the permission of the Polish government to do that; have a look at the 1951 act on Polish citizenship:
Art. 11.
1. A Polish citizen is permitted to obtain foreign citizenship only after receiving the consent of Polish authorities.
I'm just trying to figure out what else I could provide to the Polish authorities to prove my case.
You almost certainly cannot prove your case: you don't have one. Not unless you can show that neither your grandfather nor father held US citizenship prior to 1962
and that they were correctly exempted from national service
and that whoever served in US armed forces did so with Polish government permission.
And am wondering if the attorneys are incorrect and if I might have enough evidence as is to give it a shot.
Here's the worrying part: so far you have met honest attorneys; however, if you keep looking, it is only a matter of time before you meet one who says "We can probably arrange this for you", charges you
lots of money and then says "Sorry but you didn't tell us that [insert tiny detail here]; if you'd told us that, we'd have told you you have no chance."