As I have stated, you can not delete emails en masse with BTMail webmail (using a web browser).
You could download and install an email client such as Thunderbird and set it up as an IMAP account.
An IMAP account synchronises with BTMail webmail and after it fully synchronises, which could take some time with the amount of emails you have accrued, you can use that to select all or a large number of emails that are in a folder, in your case the Inbox, and delete them en masse.
In future I would suggest that you start a filing system for your emails. The Inbox is not a storage folder and it is not unknown for an issue to crop up which irretrievably empties the Inbox and any emails that have been "stored" there are lost.
You mentioned your "assistant" which implies your may be a business of some sort. If that is the case, BTMail is a residential email service and is not meant to be used as a business service. It does not have the same service level agreement as a business service and should you have any problems with the service you would not receive the same assistance and possible compensation a a business service would provide.
