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The following article should be of some help to you: https://www.gardeningknowhow.com/edible/vegetables/squash/squash-fruit-falling-off-the-plant.htm

This is typical behavior for a squash plant. They will produce mostly male (non-fruiting) blossoms early in the season and will gradually start to produce more female blossoms as the season progresses. The male blossoms typically drop from the plant and seemingly disappear. Give it some time and you will see it start fruiting.

This article may help: https://www.gardeningknowhow.com/edible/vegetables/squash/when-to-harvest-squash-best-time-to-pick-winter-or-summer-squash.htm

It sounds more like you had mixed seeds. Along with the standard green variety, you may also run across yellow, orange and white acorn squash varieties.

The side of the squash that touches the ground can be yellow, this is common.

If the entire squash is soft and yellowing, they are likely overripe.

The fruit may be staying small due to a few reasons. Environmental conditions, length of time the plants have been in the ground, the type of seed you are growing, all can effect the size of the fruit on the plant.
Here is a great article that helps you determine when to harvest.

https://www.gardeningknowhow.com/edible/vegetables/squash/how-and-when-to-pick-acorn-squash.htm

Volunteer plants can happen quite often in composting.
You may certainly get some squash on the vines, but keep in mind that they are likely from a hybrid seed. The vegetable may not resemble the original vegetable that you had planted.
Here are some links to help you.

https://www.gardeningknowhow.com/edible/vegetables/squash/growing-squash-in-containers.htm
https://www.gardeningknowhow.com/garden-how-to/seeds/f1-hybrid-seeds.htm