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Generally, cucumber mounds are spaced about 3-4 feet apart with the plants themselves thinned down to about 3 per mound. As an alternative, you may want to consider growing them along a fence or similar structure. The following articles should be of some help to you:
https://www.gardeningknowhow.com/edible/vegetables/cucumber/tips-for-growing-cucumbers.htm
https://www.gardeningknowhow.com/edible/vegetables/cucumber/planting-cucumber-fence.htm

Amounts vary for compost use, and it is possible to use too much. However, an application of about two to three inches of worm castings to your soil as a top dressing should be ok.

This article should be helpful with your upside down gardening: https://www.gardeningknowhow.com/edible/vegetables/vgen/vegetables-grown-upside-down.htm

This article should help: https://www.gardeningknowhow.com/garden-how-to/seeds/when-to-transplant-a-seedling-plant-into-the-garden.htm

If you have not read it already, this article should help: https://www.gardeningknowhow.com/edible/vegetables/cucumber/tips-for-growing-cucumbers.htm

Hills are normally spaced between 3-4 feet, but those grown in cages or on trellises can be planted slightly closer.

Hilling is a practice that is not really recommended anymore. It has become a garden habit that is more a personal choice than a "have to do it".

The benefits are that it was thought that the hill would make the soil a little warmer and a little drier. It is kind of like a tiny raised bed in the garden. But, if you are already growing in raised beds, the benefits pretty much go away.

The negatives are that the hills can dry out, leaving the seedlings with too little water. Even later, water can run off the hills rather than being absorbed into the soil of the hills. They also are a bit more susceptible to certain kinds of pests.

Really, what it comes down to is what works best for you. If you have not had any trouble growing vining plants at ground level, then keep doing that.