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Here are a number of tips on starting seeds that could be useful to you: https://www.gardeningknowhow.com/garden-how-to/seeds/cheap-seed-starting-how-to-germinate-seeds-at-home.htm
Also you need to be aware of damping off, which destroys many seedlings. https://www.gardeningknowhow.com/garden-how-to/seeds/what-is-damping-off.htm

Typically leggy seedlings are due to too little light. If you can get them more light, perhaps by moving them to a brighter location or with supplemental light, then they should resume normal growth.

I can't tell what kind of tree you've planted, but you need to consider whether the sapling has the correct amount of water, if the soil is the kind the species needs, the amount of light is correct. Also consider things like the process you used to plant the tree, the possibility of disease or pests infecting the plant before you planted it, or soil-borne organisms that are attacking the roots. I'm giving you an article about planting a container-grown tree, which would apply generally. https://www.gardeningknowhow.com/plant-problems/environmental/learn-how-to-avoid-and-repair-transplant-shock-in-plants.htm

Planting a single bloom variety should yield mostly single flowers; the double bloom varieties always have a percentage of blossoms that come out single, but some varieties have a smaller percentage than others. There's some indication that the double-flowered seedlings are sturdier and lighter in color than then singles. Here are a couple of articles: https://www.gardeningknowhow.com/ornamental/flowers/stock/growing-stock-flowers.htm
http://home.howstuffworks.com/annual-stock-flowers.htm

Hello,

Thank you for sending us your gardening question.

I would say that transplanting plants from flats directly to their permanent pots is ok. It sounds like this practice has been serving you well thus far. The plants will thrive and become strong in their permanent pots so long as they are given proper nourishment (healthy amounts of water and fertilizer).

Please let us know if you have any other gardening questions and happy gardening!

Thanks
Gardening Know How

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It sounds like you have leggy (tall and floppy) seedlings. As the article below will discuss, you should move your seedlings closer to a light source and lightly brush your hand over the tops of the seedlings (or have an oscillating fan gently blow on them a few hours a day) to strengthen and thicken them. It also won't hurt to carefully add more soil around them - if you have room. You can also transplant them into a bigger plant if they are in dire straits of tipping over completely.

https://www.gardeningknowhow.com/garden-how-to/seeds/leggy-seedling.htm