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Peas would be difficult to grow upside down as they have the tendrils that cling to things and climb upwards. Growing pears up strings and fences is a popular method and is space saving.

It is iron chlorosis, but typically, the plants get it because they simply can't get to the iron in the soil. Given what you described, I would say either you have too much phosphorus or the pH is too high. This article will help with fixing that: https://www.gardeningknowhow.com/plant-problems/environmental/leaf-chlorosis-and-iron.htm

It's hard to say for certain what or who may be the culprit. However, for most bug pests, neem oil is an effective treatment. It is also safe for people and pets, as well as friendly insects that don't eat your plants, and can be used on an organic garden. Here is more information:
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

These articles will help determine if the problem is due to slugs or snails, and if so, what to do about it: https://www.gardeningknowhow.com/plant-problems/pests/facts-about-slugs-and-how-to-kill-garden-slugs.htm
https://www.gardeningknowhow.com/plant-problems/pests/organic-snail-control-how-to-control-garden-snails.htm

The following articles should be of some help to you:
https://www.gardeningknowhow.com/edible/vegetables/peas/how-grow-peas.htm
https://www.gardeningknowhow.com/edible/vegetables/beans/tips-for-growing-beans.htm

They have simply come to the end of their growing cycle.

Due to their need to climb, peas normally do not grow upside down well.