Hello AGH,
1) “If I have an existing dropbox folder and wish to encrypt all the files on it, will making the dropbox folder a secured folder work?” Yes, making a the synchronized local copy of your Dropbox folder will encrypt all the files in it. You already have the Trial, so just try it out!
2) “Will the files be encrypted on the cloud or just my local copy?” They will be encrypted locally, and they will then be synchronized by Dropbox to the cloud, uploading the encrypted files and deleting the no longer existing unencrypted files. AxCrypt only works on local files.
3) “How does encryption/decryption work across cloud based storage and multiple users of that cloud (dropbox/gdrive)?” Not quite sure exactly what scenario you are asking about, but AxCrypt only works locally and any actions towards the cloud is handled by the cloud provider synchronizing agent. Once again, use the Trial to try it out!