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I don't normally recommend specific places for purchase, but I believe that Monrovia may have a very thorough selection of colors and some in your range that you would find appealing.

Unfortunately, that will be a very hard thing to pin down. You could try taking samples to your local extension services. Depending on what tech they have available to them, they may be able to run it through CRISPR or a sequencer and find out what it may be.

https://www.gardeningknowhow.com/extension-search

It definitely needs protection at that young age and in a container. It should do fine in your garage. Water sparingly, about once a month, just to keep it alive. Too much water can cause rot.

Likely, they are seedlings from your shrub! They may not look like the original at all, or they may resemble it quite closely. This happens as they drop seeds. They can reproduce, both, sexually and asexually.

Before transplanting, I would wait until winter dormancy. This will ensure that there is no damage done, and they survive transplant. This article will explain how to do so:

https://www.gardeningknowhow.com/ornamental/trees/crepe-myrtle/transplanting-crepe-myrtle-trees.htm

I think that this is related to problems with your mandevilla. Here are some articles that will help:

https://www.gardeningknowhow.com/ornamental/trees/crepe-myrtle/crepe-myrtle-not-leafing-out.htm

https://www.gardeningknowhow.com/ornamental/trees/crepe-myrtle/crepe-myrtle-care.htm