Coding an app is at this point I think beyond me, unfortunately!

In terms of how I use ebird and merlin in the field...I think it might not work well for you because I do rapidly look between bird and phone.

Something that might work better for you is a strategy I know is used by some older birders from the days before smartphones, which is to carry a small audio recorder around with you when you're out birding. When you see a bird, stare at it for as long as you need to, while talking quietly to your audio recorder about all the details you notice about the bird. And if you see/hear any birds you can ID in the field, you can mention that into your recorder as well.

Then you can review your saved audio at a later time, when you're at home with your field guides, to figure out what it was you must have seen. And then using all your bird IDs, both the ones you managed in the field and said into the recorder, plus the ones you figured out at home, create an after-the-fact ebird list.

That way you don't have to spend much time using your close vision while you're in the field!