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This is due to a lack of light. This article will help: https://www.gardeningknowhow.com/garden-how-to/seeds/leggy-seedling.htm

1) I would not try and repot the plants, as moving them would only shock the roots and they likely would not survive. As long as there is anough space in the container and they have adequate drainage, the plants should be fine. 2) Normally, you would thin the seedlings down to your strongest plants. For instance, your healthiest tomato, best cucumber, and melon. I would not have more than three of these growing in one container. For more infomration on thinning, this article will help: https://www.gardeningknowhow.com/garden-how-to/seeds/thinning-seedlings.htm 3) Most fertilizing is done at the time of planting, usually amending soil with compost is sufficient. However, if you apply any during the growing season, a fertilizer higher in phosphorus works best.

It sounds like they are experiencing cracking, which is uncommon with cucumbers but possible. Cracking is normally caused by irregular watering- going through dry period and then a heavy watering. The plant, being water starved, sucks up the water rapidly and over fills its fruit with water. Small cracks form (you can kind of think of them as stretch marks on fruit) and they heal quickly, but leave scars that get bigger as the fruit grows. The fruit tastes fine and is fine, but looks a little marred. Making sure they are evenly watered will correct the problem on future cucumber fruit.

This article will help: https://www.gardeningknowhow.com/edible/vegetables/cucumber/tips-for-growing-cucumbers.htm

No. This article will help explain the cross pollination process: https://www.gardeningknowhow.com/edible/vegetables/vgen/cross-pollination.htm

Unfortunately, there is no cure or treatment for cucumber mosaic (CMV). Prevention is all you can really do. Infected plants need to be removed. Here is more information: https://www.gardeningknowhow.com/edible/vegetables/cucumber/cucumber-mosaic-virus.htm