Editing an already imported module

Use the -Force command with the Import-Module and it will reload it.

Once a module has been imported, changes to it are not recognised since the module is loaded into memory. However, I've always been able to do a Remove-Module foo, followed by an Import-Module foo to load new functions.

All that said, your PSD1 file doesn't look right. It should have a ModuleToProcess field set to 'MyModule.psm1'. Then when you do Import-Module MyModule or Import-Module .\mymodule.psd1, PowerShell will find & load the associated MyModule.psm1 file. I wonder if that is causing you to run afoul of some caching PowerShell does?