So many changes to your citrus that it will be hard to narrow it down to just one thing.

When you say 'added Epsom salts', how did you apply and how much?

Moved into sun from where? Indoors? or from a shadier location outdoors?

Regarding bitter apple, I think I used that once on something and all the leaves burned. I will not use it again.

Did you fertilize recently as well?

I think it was spraying bitter apple (vinegar's cousin) on the tree's foliage was the major concern.

Doing too much for a young tree is not necessary, sometimes less is better. When transplanting, tree need some adjusting. A couple of weeks out of direct sun is preferred.

I agree with everyone above I think it’s quite a lot done to it the spray is of much concern to me I think it burned your tree

The "take off" was due to being moved from a perfect citrus potting soil to a bonsai mix. Blooms are triggered by temperature and water availability. Producing flowers and fruit is natures way of continuation of the species. It has come from a heat wave to cooler temperatures which are triggering the flush. Had the plant had the original potting soil it would have flushed in the spring.

You added magnesium. Why? Did you see an inverted "V" of green at the base of the leaf (particularly older leaves)? Mg is necessary for chlorophyll formation and seed development. It is more of a problem with sandy soils. It is usually corrected with a foliar spray. Quickest way to kill a citrus is add elements helter skelter.

Growing indoors you have to mimic the outdoor seasons. February not much happens. The largest flush is the spring with the second largest the summer and smallest the fall.

The leaves for what ever reason do appear sunburned. If the leaves want sun they will point towards the sun (like first photograph). Second photograph the leaves are pointing downward (blocking sun). Savvy citrus groves use netting to block some of the suns rays.

Limp growth is a sign you are underwatering or your potting mix is not holding enough water.

Plant came with enough fertilizer for 2-9 months of growth. Don't fertilize unless you see signs you need fertilizer. Don't fix things that aren't broken.

With trace elements (micro-nutrients) you need to be very careful. Citrus need small trace amounts of boron, magnesium, and molybdenum in the soil. You definitely didn't have an Mg deficiency but as a reference point to help you know if you gave your plant a toxic dose, one reputable website I am on suggests for deficiency you give one pinch per tree per year in 4.5 L of water. Larger doses can be lethal. I know with hybrid tea roses I use to give them a lot of Epsom salt with little drama. I remember experimenting with Epsom salt a few years ago with potted citrus and the dose I used was toxic.

On the subject of making your own potting media. If you can't measure it you can't manage it! Can you measure the key characteristics of your hand blended mix?

1) Total Porosity

2) Bulk Density

3) Air-Filled Porosity (AFP)

4) Water Holding Capacity

5) Wetting capacity

6) Shrinkage

7) pH

9) Electrical Conductivity (EC)

10 Cation Exchange Capacity (CEC)

Does your mix contain the macro-nutrients N, P, K, Ca, S and Mg?

Did you add the micro nutrients B, Cl, Mn, Fe, Zn, Mo and Ni?

Did you add the micro and macro elements in the right ratio?

Growing citrus is like solving a Rubiks cube. Random turns never solve the puzzle.