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I'm not sure what went wrong with your Magnolia; but understand the stress of losing a new planting.
A consistent watering regimen is needed immediately following planting to ensure your new tree remains healthy and adapts to its new home.
This article will help you review the care.
https://www.gardeningknowhow.com/ornamental/trees/magnolia/magnolia-tree-care.htm

You have a bad spidermite issue, it sounds like! This is devastating, and usually fatal by this point.

You may be replacing the tree soon. In the future, you will want to catch them as soon as you see signs of spotting or webbing. This article will help: https://www.gardeningknowhow.com/plant-problems/pests/insects/spider-mite-treatment.htm

The crushed stone is likely not harming or killing the tree. It's not helping the tree like the natural leaf litter, or a mulch layer of organic matter would.

One important consideration is that the soil base is not compacted, and that the rock is not driven over with vehicles or otherwise compacted into the mud layer, forming a mixed aggregate concrete-like layer that interferes with water and air movement into the upper soil profile.

Unfortunately, this is the cause. You can't use un-composted ashes, no matter what the source. From this point, I would add something such as iron sulfate, or wettable sulfur to bring the pH of your soil back down... I would imagine that it is well up into the 8 point range.

Ashes, when added to compost as it is composting, will help add trace minerals and some macro nutrients.

Here is a composting guide to help you get started: https://www.gardeningknowhow.com/composting/basics/ultimate-beginners-guide-composting.htm

This article will help with the care of Magnolia trees: https://www.gardeningknowhow.com/ornamental/trees/magnolia/magnolia-tree-care.htm

Without seeing the mushroom in question, it would be very difficult to know if it was the cause. Mushrooms can have beneficial relationships with trees, no relationship, or parasitic relationship, so knowing the exact type will be necessary to know if it is causing any damage to your trees.

Wet weather can cause fungal infections, which can be the case if you have had more rain that usual, or if the tree was kept too moist upon planting. You can use a 1/2 cup of DOLOMITIC LIME and a tablespoon of WETTABLE SULFUR to kill off infections, and once per year to prevent them.

You may also resort to spray fungicides. This article will help: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

This article will give you more information on Magnolia care: https://www.gardeningknowhow.com/ornamental/trees/magnolia/magnolia-tree-care.htm