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Unfortunately there is some bad advice circulating about coffee grounds. They do not substantially lower the pH for acid-loving plants because whatever acid the beans had originally mostly comes out in the coffee liquid. And not all coffee beans are acidic to begin with. Your best bet is to obtain a potting mix designed for orchids. In addition to having a low pH, it will be a fast-draining mix which orchids must have. Here are two articles on general orchid care: https://www.gardeningknowhow.com/ornamental/flowers/orchids/indoor-orchid-care.htm
http://www.aos.org/orchids/orchid-care.aspx

The best ingredients are well composted materials that are specific to the plants that you are trying to plant at the time. Although an all purpose potting soil is just fine, the best soil for your application will take some research on your part.

You will have to find out what the plant grows in naturally and mimic that to the best of your ability.

Simply put, this does not have a simple answer, as all plants will have different requirements.

Despite the wonders of coffee grounds touted on the internet, the scientific evidence is that coffee grounds (and filters) are fine for the compost pile. That's it. They do not stop slugs or acidify the soil. To begin with, the acid value of coffee BEANS varies quite a bit depending on where they are grown. More importantly, most of the acid leaches into the coffee liquid when beans are brewed. If you need to acidify your soil, seek a source such as sulphur that has a known amount of acidity so you can dose your soil correctly. Start with a pH test so you know how much to add. This article goes over some of your options.
https://hortnews.extension.iastate.edu/1994/4-6-1994/ph.html

Immediately in the soil, they are only good for aeration until it fully breaks down. Then it will add potassium, phosphorus, calcium and carbon into the soil. This will benefit most plants, no matter what kind. This makes a wonderful soil amendment. Here is an article that will help: https://www.gardeningknowhow.com/composting/ingredients/eggshells-in-the-garden.htm

In areas with compacted soil that retains water, working conifer needles into the ground will increase aeration, which improves drainage. For quickly draining soils that cannot retain adequate amounts of water, adding conifer needles increases organic matter, giving moisture something to hold on to and allowing plants more time to absorb it.
This article will help you.

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/how-to-improve-clay-soil.htm

It would work at first, but as it rots, there is no guarantee that it won't clog up. As long as there is plenty of compost or other means to aerate the soil, then some hay won't hurt. Some even plant directly into hay bales, but this can lead to infections quite quickly.