You can edit the images from your external on your iPad, the issue is that those images are uploaded to the cloud and the edits are not saved to the images on the HD but to the images in the cloud. This is a feature I and many others would like to see - to be able to edit the images on the HD itself on the iPad. I've added it to an existing feature request. I had a student I helped with this same exact workflow last week.
I recommend you upload your images to LR. I use my phone and LR on my phone set to automatically import all images from my camera roll. Get the images uploaded to the cloud, do your edits on the iPad and then you simply sync to LR Classic. When you sync, the originals are downloaded to your preferred location. Some will stay in the cloud, some will be just the smart previews - it's based on how much space you have in the cloud - smart previews don't count against you. But, you will see all originals downloaded. You set up where they download in the LRC preferences. I have mine sorted by decades so I only need to update the location every 10 years.
Hopefully that makes sense, if you have more questions, ask.
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