Any free codes to rollback Europa Universalis 4 and other Paradox games without using their official forum?

Unfortunately there isn't.

The reason why they have that procedure in the first place is explained in this Steam Community post. All the Paradox games before October 1st 2018 were not compliant with the new EU GDRP. They used the player's personal information in ways which now required explicit consent, and those older versions didn't include a way to ask for that. So the regular way to downgrade a Steam game - by abusing the beta program functionality and make each past version a public beta - was no longer legal.

Their way to get the permission of a customer and to prove to the authorities that they indeed have that permission is by requesting a key for a private beta invite through their forum system, which allows them to show the player their privacy policy.

Note that this key unlocking procedure only applies to game versions released before they became GDPR-compliant. According to this forum post, the following Paradox games should be available on Steam as public betas from these versions on:

• Europa Universalis IV: 1.27 and onward
• Hearts of Iron IV: 1.5.4 and onward
• Stellaris: 2.1.3 and onward
• Crusader Kings: 2.8.3.4 and onward