[SalesForce] Redirecting to external site from the value from url field

It depends on the OAuth2 implementation that you're authenticating against, but generically speaking, login.salesforce.com should work for production instances, test.salesforce.com should work for sandbox instances. What happens is that when you hit either login server, if there's a path afterwards, the login server tries to forward them to their existing session.

For your managed package, your callback URL should look like this:

https://login.salesforce.com/apex/myns__mypage

Or for sandboxes:

https://test.salesforce.com/apex/myns__mypage

Since most OAuth2 implementations only allow one callback URL, you'll need to have two separate "apps", one for production, and one for sandboxes.

Your VF page should query the Organization object beforehand to determine if you're in a sandbox or not.

You may want to implement the string replacement code found in both of those answers you referenced. I've run into similar issues generating presigned URLs for S3 and learned the (very) hard way about URL escaping reserved characters in the policy. In your code, you may want to try something like this:

...
String policyEnc = EncodingUtil.base64Encode(Blob.valueOf(strpolicy));
policyEnc = policyEnc.replace('+','-').replace('=','_').replace('/','~');
...

Other languages (eg., Python in my case or .NET/Java in your case) seem to do a better job with this type of URL generation, but I've spent countless hours trying to debug this stuff in Apex in the past. Hope that helps!

Edit: just to be clear, the reason this should help (if not solve the issue) is because you're generating a presigned URL in the end, and that URL has to be properly formatted. If your base64-encoded policy includes characters normally found in URIs/URLs (eg., +, =, or /) it will fail.