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https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/hydrangea-propagation.htm

It depends. This species of hydrangeas breaks dormancy first by only growing new stems from the crown/base of the stems. About 1-2 months later, the leafless live stems break dormancy separately and leaf out. Because the leaf out time can be quite late in some years, consider not pruning the leafless stems until the end of June in Ohio. If you prematurely prune, you may be reducing or eliminating the spring blooms. However, the second flush occurs in the summer/fall once the new stems that started growing in spring 2024 attain sufficient height to trigger flower bud set and immediate blooming. Dormant flower buds begin to develop inside stem endings close to the time that the last flower bud of each flush opens.

I'm seeing signs of disease in the Mahonia, as well. Bacterial or fungal diseases are common in wet soils, and they can be hard to identify by pictures. The best way to identify the exact issue will be your local agricultural extension service.

In the meantime, treatment will focus on maintaining the disease. Fungicides and good sanitary practice will be your best bet. This means keeping any dead or dying debris pruned away and removed from the area.

https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/growing-hydrangeas-hydrangea-care-guide.htm

https://www.gardeningknowhow.com/ornamental/shrubs/mahonia/growing-leatherleaf-mahonia.htm

This is not true for every situation. They are picky bloomers. Aluminum sulfate is usually only used to adjust pH and influencing the bloom color. In most cases, this is not a preferred way to lower the pH since it stays around in soil for a very long time, and becomes toxic. Sometimes this goes to the point of halting all growth, though is takes quite a long time.

I would say that the pH of the soil was not where it should have been, or that there was an excess of unavailable nitrogen and bound up phosphorus. Another thing to consider about phosphorus interaction with aluminum sulfate would be that, eventually, it binds out phosphorus. This makes it unavailable.

Every situation is different, so it is best to always test your soil before doing anything to it. This will tell you exactly what the soil needs or doesn't need.

Your local extension service can help with more accurate results than home tests.

https://www.gardeningknowhow.com/garden-how-to/info/what-is-extension-service.htm

It depends on how close to warm summer temperatures you are and if close, do you have time to test soil moisture almost daily and monitor afternoon wilting episodes.