The Maintenance dialog relies on the ARPNOREPAIR property value as well, that's why when you set this property, the repair option is disabled in both places.
First of all, you should think carefully whether you really want to disable repair in one place, and leave it as is in another one. If that's truly the case, you should modify the Maintenance dialog the way you'd like (for instance, change the condition which disables the Repair button) and inject this modified dialog into the UI sequence.
This might sound pretty scary for beginner, that's why I suggest you starting from the "Customizing Built-in WixUI Dialog Sets" article in WiX.chm file.