What's your question? Ask

Please let us know what the bug looks like. This will help us with giving you the best advice on how to get rid of it. Normally, however, neem oil can take care of most insect pests.

Do you know the variety of lemon? That will make a difference in our answer.

We would recommend that you get a citrus specific fertilizer. They are a little higher in acid, which lemon trees need. Here is more information on citrus fertilizers: https://www.gardeningknowhow.com/edible/fruits/citrus/fertilizing-citrus-trees-best-practices-for-citrus-fertilizing.htm
https://www.gardeningknowhow.com/edible/fruits/lemons/learn-about-fertilizer-for-a-lemon-tree.htm

Have you fertilized? Nitrogen encourages leaves to grow, but not fruit. Too much can actually keep a plant from producing fruit. This article has information on how to fertilize your trees: https://www.gardeningknowhow.com/edible/fruits/citrus/fertilizing-citrus-trees-best-practices-for-citrus-fertilizing.htm

The soil may be lacking phosphorus. Add bone meal and have your soil tested. This article will explain more:
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/phosphorus-plant-growth.htm

The requirements for lemons are very similar to that of limes. If you have not read it yet, this article may also be helpful: https://www.gardeningknowhow.com/edible/fruits/lime/reasons-and-fixes-for-lime-tree-not-producing-blossoms-or-fruit.htm

This article should help you: https://www.gardeningknowhow.com/edible/fruits/lemons/learn-about-fertilizer-for-a-lemon-tree.htm

With any tree, they will not cause damage to walls or other structures unless they lack for something - either room for roots or water. I think certainly a dwarf citrus or dwarf magnolia would be fine as they have very small root systems compared to other trees. A angel trumpet (assuming you mean brugmansia) may get a little large, but I am uncertain how much space you have so I cannot say for certain.