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If the stems are still alive, they will regrow their leaves. I would give them a month, and if you do not see growth by then, they are most likely dead. These articles will help you as well:
https://www.gardeningknowhow.com/plant-problems/environmental/how-to-tell-if-a-plant-is-dead-and-how-to-recover-an-almost-dead-plant.htm
https://www.gardeningknowhow.com/plant-problems/environmental/tips-for-saving-cold-damaged-plants.htm

Yes, neem oil can be effective in treating these pests.

Whether white flies or aphids, treating with neem oil should help take care of the problem. Neem oil is effective against many types of insect pests and is perfectly safe as well. Here is more information: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

Photinia is very susceptible to the fungus leafspot, called Entomosporium, that can defoliate photinia plants. Plant photinia in full sun and provide good air circulation. Remove and destroy fallen leaves, prune off diseased twigs. Avoid overhead irrigation. Fungicide sprays (copper-based products) are also generally necessary for this disease, beginning from bud break and applied every ten to fourteen days until all foliage has matured. Mature foliage is no longer susceptible to infection.

As for the Oleander, water when the soil is dry (about an inch weekly). During droughts, water a couple of inches each week. Keep in mind, however, that while the soil may not need much water, the leaves and petals can dry out quickly. Mist the leaves with water (early in the morning) to avoid this, especially after a hot, sunny day.

These are aphids. The ants are attracted to them. The neem oil will work on them, but it takes a little time for neem to work. This article will also help:
https://www.gardeningknowhow.com/plant-problems/pests/homemade-aphid-control-a-natural-way-to-kill-aphids.htm

It sounds like you may have both pests (that oily sheen could be honeydew from them) and a fungus. I personally like to use neem oil because it is organic and also kills both pests and fungus in one go, so you don't have to use as many products. Here is more information on it:
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm