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It would seem that you have a severe pH issue. Dolomitic lime can help here. This will help the pH return back to a more suitable level.

Here is an article that will refresh you on the care of these: https://www.gardeningknowhow.com/houseplants/areca-palm/growing-areca-palm-indoors.htm

From the way it is progressing, I would say that your mix is a little too "hot". This means that it might contain too much fertilizer for the palm. I would repot into a palm mix, with just a little of the Innes3. This will need to be done fairly quickly, as this will die in a very short time period if not taken care of.

These are scale insects.

I would treat with Neem Oil, and you may need to treat several times.
This article will help you.

https://www.gardeningknowhow.com/plant-problems/pests/insects/control-plant-scale.htm

All of these pictures are very vague, as sometimes it is hard to tell what a plant is without flowers. It looks like the last two are dracaena and the second picture look s like a mulberry. I can't even begin to tell what the first picture is. If you want a definite answer, you can take samples or pictures to your local extension service, or agricultural center. It is just difficult to tell from the pictures.

No, it is not likely the light. unless it remains in direct light for long periods, and even then there is a bigger problem.

It looks like you are using heavy clay soil. This will be the cause of the issues, and could kill the tree eventually. It will be best to use a mix of potting soil and compost with a little sand added. This will be light and airy which will allow the roots of the tree to breathe.

For more information on the care of this palm, read this article: https://www.gardeningknowhow.com/edible/vegetables/squash/growing-butternut-squash.htm

If you sprayed the green foliage or any exposed, cut roots, or if you cut into the palm trunk and then sprayed the exposed tissue with Roundup herbicide, a week ago or more, and all the leaves are turning yellow, it is not good news and you may lose the palm.

But if you sprayed some herbicide on the external surface of the brown tree trunk, then it would not likely affect it and there may be another cause for the yellowing.

If all the leaves are yellowing, that's not good. But if it's only the lower palm fronds, then it may be natural yellowing and shedding of the oldest fronds, and just a coincidence with your herbicide spraying.

Drought stress could be another reason but not as likely.