Didn't have time to read your entire post. But that looks like sunburn to me. Leaves may continue to manifest sunburn over next few weeks.

Never place a plant in full sun if it's been in shade over winter. Move them gradually over weeks. Those leaves won't 'recover'. Hopefully your plant will continue to grow.

Your baby will be fine. I agree with Dave †hat it is most likely sunburn. You brought it out into full sun after only a few days when it should have been after a wee or two.. You should should start acclimating it to direct sun all over again by putting it in the shade for a week, then partial shade or sun for a few hours a day for another week and then full sun. There is no need to remove the damaged leaves.

Be very careful about overwatering. Overwatering is the number one killer of potted citrus.

You have been given good advice. I would add one thing to think about/change... in cool climates terracotta pots create cool soil due to evaporation through the clay; and citrus roots do not like to grow into cool soil.

In hot climates, at times the cooling effect is a positive thing; but in hot climates, citrus do better planted in the ground.

For climates like yours, where you have to bring the plants inside for the Winter, I would opt for plastic or ceramic or polymer pots.

As for your damaged leaves, they will not get better; but they are still producing energy; so leave them as they are.

Looks like sun damage alright.

I have found that transitioning overwintered plants outdoors in the spring without damage can be a challenge. Some handle it better than others. Just remember that even dappled shade is many times brighter than a sunny indoor windowsill. Full sun can be a big shock to a plant that has grown accustomed to a living in a dimly-lit home. Start them in a semi-shaded area, or use a screen/shadecloth, before you expose them to full May/June sunlight.

Thank you, everyone. I appreciate your responses. I did think it looked like sun damage but I didn't realize that the damage can continue to appear well after the plant has been removed from the direct sun - thanks for this insight.

My plan is to repot Larry (the plant) into a smaller/lighter pot with some proper soil mixture and to slowly introduce him back to the sun. I think the pot we have is too large anyway and my husband recently injured himself and can't help me move it - maybe why I rushed his transition to the outdoors over a few days as opposed to a week or two. We're at high elevation and the sun is very powerful here in CO so perhaps moving extra slow is even more important.

A few lingering questions...

1. Does anyone have a conversion chart or recommended formula for adding fertilizer to such a young/small plant? The "dosage" chart given on the bag of citrus fertilizer I have seems to be for much, much larger trees and I want to make sure I don't overfertilize.

2. Any ideas as to what the initial few white spots/defoliation might have been from? Those spots did not progress like the sun burn spots and happened several weeks before moving him outdoors. They are shown in the first image.

Thank you again! I am happy to hear that Larry should recover.

I'm glad to see I'm not the only one who names her citrus :-). I hope Larry feels better soon!