Hello,
1. I would like you to attach a troubleshoot file to your question. Please, replicate the issue, then access router's WebUI, go to System -> Administration -> Troubleshoot section and download troubleshoot file from there. The logs in the file might provide more insight into the issue.
2. Backup compatibility is defined not by the batch number, but by router's product code. Backups from devices with different product codes will simply not upload. Alternatively, you can try to copy configuration files only from the backup and move them to the other router using scp/WinSCP, as instructed in this page. The directory with the configuration files is /etc/config, excluding system file.
3. Did you try configuring a different router's HTTP port in System -> Administration -> Access control page?
4. No such plans at the moment.
Best regards,