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If amending the clay soil is not an option, plants such a low growing viburnum's, euonymus, juniper, cranesbill or salvias may do well. These articles should help

https://www.gardeningknowhow.com/special/xeriscape/xeriscape-landscape-design-ideas-for-clay-soil.htm

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/plants-for-alkaline-soil.htm

This article will help you. It recommends Clearex Salt Leaching Solution and/or leaching with water:
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/reversing-soil-salinity.htm

Salt can be leached (carried) out of soil by large amounts of water. Make sure you water deeply each time you water your lemon tree, so that the water will carry the salt below the root zone, deeper into the soil. If you water shallowly, salts will build up near the soil surface and affect the roots. Over the course of weeks or months, the salt level should decrease. Also, if the water itself is salty, make sure it doesn't get on the leaves.

This article is written for Florida, but also applies to any salty soil:
http://edis.ifas.ufl.edu/ae171

You may amend the soil with things such as perlite, worm castings, and coconut fibers. This will significantly improve the quality of your soil. Here is an article that will give you more information: https://www.gardeningknowhow.com/special/organic/how-to-easily-improve-your-clay-soil.htm

This is a complicated subject. It really depends on where the soil came from and if there were cats, or dogs that used the soil.. If so, then this would be considered unsafe to use on anything that you would consume.

To be on the safe side, I would only use it for lawn maintenance, and inedible garden beds.

Your first step is to get an ID on the insect. Get a good picture of one and post it one of the insect ID websites. Once you know what it is, you can research proper treatment. Without an accurate diagnosis, you won't find the correct treatment.