Q: What can I do if the editor does not confirm my withdrawal request?

If the journal is a reputable one, I think you can wait a while longer before you decide to withdraw. Some journal editors are not very responsive to emails: possibly, they are very busy and do not have the time to reply. I would suggest that you send emails more frequently, maybe once every week for the next 3-4 weeks. Be very polite in your emails, and explain that you are anxious about your submission and a reply from their end would be appreciated. If there is still no response nor a status change by the end of this month, you can consider withdrawing your manuscript.

However, as you have rightly pointed out, the withdrawal process for the manuscript will not be complete unless you receive a confirmation email from the journal editor. In case the editor does not reply even to your withdrawal request, you will need to send multiple reminders and wait till you receive a reply. If there is still no reply, you will have to write to the journal saying that you will consider the withdrawal process complete if there is no reply within a specific time (set a deadline of 2-3 weeks). You can then submit your manuscript to another journal. However, make sure you write to the editor of the second journal, explaining the situation and providing the email thread about the withdrawal as supporting evidence to assure them that there was no intention of duplicate submission from your end.