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The tree ranges from about 6-10 feet tall with a similar spread, making it suiable for use as a hedge, unless you choose to maintain its spreading growth through pruning.

There is no sure way to encourage spreading in a particular manner.

You want to make the container about 5"-10" wider than the rootball of the plant you buy and at least 2' deep.

You may like a dwarf Korean lilac. They spread nicely, but stay small so they would not outgrow the container too quickly.

It is likely a phosphorus deficiency. Fertilize it with a phosphorus rich fertilizer to help it bloom. This article should help as well; https://www.gardeningknowhow.com/ornamental/shrubs/lilac/lilac-bush-not-blooming.htm

The soil could be a factor too. Try amending your clay soil to see if this improves the issue. This article will help with that: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/how-to-improve-clay-soil.htm

You can do something called rejuvination pruning. It can be done once in awhile without killing the shrub. This article has direction on how to do it at the bottom of the article:
http://www2.ville.montreal.qc.ca/jardin/en/info_verte/lilas/types_taille.htm

There are a lot of things that could be affecting the lilac: borers, canker and root rot to name a few. They are generally tough plants and come through just about any adversity. I would try cutting it back. If it survives, it will not bloom next year, but blooms should return the following year.

I have never heard of a lilac changing its color. My best guess is that it was due to the late snow. Now we are both baffled. Hopefully someone else may be able to offer some insight on this.. Please post again next year and let me know what color it blooms !!!