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My opinion is that there are three most important factors to "grow a decent garden":

Growing conditions, growing conditions, growing conditions.

Start with soil testing and recommended amendments.

https://www.wikihow.com/Prepare-a-New-Garden-Bed

Water management is the next consideration.

Give us a hint of what you want to grow, how much land area, and existing conditions that are challenging your gardening know how. Photos are good. We will try to help.

Covering the area with cardboard or clear plastic will suffocate the weeds, but works best when the weather is warm. It also works well to use glyphosate to kill whatever is green and growing. Since you aren't in a hurry, you could do the cardboard method first, then when you are ready to work on it, you can use glyphosate if any weeds remain growing.

There will be no disinfecting enough to keep the virus at bay. Usually, once it is established it will not leave. If there are other plants nearby, then they can carry the virus. Many plants will not show symptoms, and will be unaffected, however, many will be permanently affected and will require that the area never contain any plant that can catch it.

There are around 150 plant species that can contract the disease, so as long as you avoid them, it won't pose any kind of threat. Unfortunately, there is just no getting rid of it once established in an area, due to the way it spreads (even on plant pollen).

This article will help: https://www.gardeningknowhow.com/plant-problems/disease/tobacco-mosaic-disease.htm

It does appear to be an Iris, but I wouldn't move them until mid Summer to late Autumn. Doing so now can harm them to the point of dying off, or prevent them from flowering this year.

Here is an article that will help you to divide them:

https://www.gardeningknowhow.com/ornamental/bulbs/iris/dividing-transplanting-iris.htm

I don't suspect that leveling the area will cause any problems to the wall. If you are going to use sand, though, I would mix it into a good topsoil. Sand, alone, can cause some issues over time.

Here is an article that will help you to level out this area correctly:

https://www.gardeningknowhow.com/lawn-care/lgen/fill-uneven-lawn-low-spots-how-to-level-a-lawn.htm

It depends on the weed killer. If you still have the container, it should tell you on the back when it is safe to plant. If you don't have it, the label can be found online. According to the article below, it generally is safe in about a week.

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/how-long-does-weed-killer-last-in-the-soil.htm