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This article should help: https://www.gardeningknowhow.com/plant-problems/pests/lawn-grubs-how-to-get-rid-of-grub-worms.htm

Most likley this would be mushroom compost. Here is more information: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/mushroom-compost.htm

Normally, it is better to plant first and then apply your mulch. However, if you already have mulch in place, you can simply move it away from the planting area, dig your hole, plant and then replace the mulch.

I have not heard of pine mulch causing such a reaction. However, if the layer is too thick, it could be choking off the oxygen allowed to get to the soil and, thus, affecting the plants. Also, if the mulch layer is too thick and the soil doesn't drain well, such as heavy clay, it could be holding too much moisture in the root zone, causing the yellowing of the leaves. I have also seen cases where the pine mulch attacted neighborhood dogs to the area, relieving themselves and spraying the plants with their urine. Thus, burning the foliage just enough to turn the leaves yellow before they die off.

Mulch (assuming here that you're talking about organic mulch) does not have the kind of texture that is good for nurturing roots. It's too open and coarse in structure, and not chemically active, and does not retain moisture in the manner that soil does. Of course, there are some very strong plants that grow when their seeds are just sprinkled on top of the mulch -- usually we call them weed. But for most flowers, you need to scrape the mulch aside and plant in the soil. This article has general info and references for mulch: https://www.gardeningknowhow.com/garden-how-to/mulch/benefits-of-using-mulch.htm

I would refill the trench with fresh dirt and compost, leaving about an inch or two for the addition of any rock or mulch after planting.