PS: Forgot to add >>
You needn't worry about replies or "signatures" being different when using different devices that are receiving emails that are being forwarded originally addressed to a single alias: yoiu can still have that "Main" email address entered in yur Outlook, but can also configure that "email account" to be included in the Group to process incoming mail for .....

For example:
1. your Main Email alias (which is the Webmail one) is "MainMan @ YourDomain.com" ..... email comes in, and Webmail immediately forwards it to "MainManLaptop @ Your Domain.com" and "MainManPDA @ YourDomain.com" and leaves a copy in webmail.
2. Your *** gets an email via Outlook addressed to MainMan @ Your Domain.com because it has been forwarded, and when you reply, you simply "Change Account" in the "From" box so it looks like "MainMan" is replying ..... it will be SENT via MainManPDA account, but it's ID will remain as if there is only one computer overall.

You just have to be sure to configure that "MainMan" (or Primary) email account to not be included in the sending/receiving group....
.