I don't know if it's the lilac impacting your retaining wall - looks like failure due to the ground heaving, shifting, and settling. Not a mason, but would think freezing and thawing (or a bad mortar mix) could cause mortar to pop. And now that it's underway, it'll continue with or without the lilac. The root system is probably fairly extensive if you're getting suckers ten feet away. If you want to get rid of it, dig around it in a circle, chopping off the roots as you go. Labor intensive, but doable. Then paint the fresh root cuts with a non-selective herbicide like RoundUp or some other stump killer type product. Or, have it it professionally taken out and the stump ground. You might still get a sucker or two, but since they'd no longer be attached to the parent, mowing over or ripping them out should take care of them. I have many, many, many feet of old and huge common lilacs -and I've never seen a sucker more than 6" from the parent plant. But I concede different varieties do different things. And I kind of welcome the close in suckering since it replenishes any old wood I might lose. There are wall and tree/shrub experts on the forum; hopefully they'll chime in.

Do you have a name for your lilac? I have had very poor luck with James McFarlane or McFarland. Miss Kim takes a LOT of water in the location I have it in. Was fine in another place I was just holding it in, never needed extra. Have to see how it does with better mulching and more root establishment since it has been in place a year now. I have heard bad things about the tiny Tinkerbelle bushes. Don't take our cold well, die after only a couple years anyway. Is your bush getting a good airflow around it? Sometimes they need more air circulation to keep problems away, like the discolorations of late summer. Being small, it could just be very close to the damp ground and not having any airflow. How much water is it getting? I try to do a good soak with bush mulched well, just every 4-5 days if we don't get rain. My dirt holds water well, lots of clay and organic material in it, not sandy at all.

Lilacs are subject to several different leaf diseases. Most of which are not serious under normal conditions. Some leaf diseases are not easily distinguishable to leaf damage caused by physiological factors. We need to know when were they planted, is this the first year you have seen this, what fertilizers, weed killers might have been used around the plant, when did you first see this. Leaf diseases that occur at the end of the season frequently are not a problem. For now I would clean up fallen leaves and take some samples from the bush and send them to your local extension service. With the weather we had this year many plants are showing problems that are not usually seen. Most of these will not be problems with normal weather.

Do you know which variety you have? Hopefully, you have one of the smaller varieties. If you want it to bloom, don't chop off the top of the plant. That's where next year's blooms come from. The preferred way of pruning lilacs is to thin out older branches by cutting them out at the base of the plant - only a couple each year. This will allow sunlight to get into the plant. You may not need to do this yet if your lilac is still young. Also, depending on how small the shrub was when you first planted it, it may take a few years to get established and reach full bloom. A long-time gardener once told me it takes 7 years for a lilic to reach it's peak. Be patient and good luck!