I'm not an expert and have problems of my own. But from what I have been reading it is possible the plant being near the window is dropping the soil temperature below 65F or so. Maybe thats what is happening and the roots are going dormant causing the leaf drop.

I guess even if the room is over 65 it is possible for the soil to get colder due to evaporation.

Check the soil temperature etc, if it has a cold root temperature it will/may chuck the leaves off if the leaves get direct sunlight.

There may be problems with the fertiliser being an orchid one, check the npk ratio and what micro nutrients it has, citrus unke a lot of plants need a lot of micronutrients, its best to change fertiliser for summer to winter, depending what the plant is doing at the time, summer feed i use 24:14:14 and winter feed 18:18:18 , the P and K look a bit low compared to all the winter feeds i have seen, all year round formula seems to be 28:7:14 and it didnt give as good results as the seperate summer/winter formula.

When you killed the scale, did you use a pesticide?, using some pesticides it has been reported that you have a vastly increased chance of a masive attack by spidermites and thrips , both are very dificult to see, check carefully for spidermites!

Starloc,

For the first cleaning, we used baby shampoo mixed with water. Since then, we've sprayed it with Biowash.

What particular winter feed do you use?

Matt

What is Bio-Wash? Other than some rather harsh products, I don't see anything on the web that ought to be used on plants. If you've used the shampoo, I'll betcha that's what did all of your plants in.

It might have even been the baby shampoo. Contrary to what we are led to believe, it is just just as strong a detergent (not a real soap) as other shampoos (which are loaded with harsh products). It is, however, pH adjusted so that it doesn't sting the eyes.

Matt, what did you use to rid scale? Baby shampoo or a chemical insecticide?
Chemicals are dangerous. Especially if used indoors. Since you're in NY, you either sprayed inside or hauled outdoors to apply..Or did you use systemic?
I once killed a Gardenia tree by using a harsh chemical, even though, the tree was outside, and I followed directions to a T. Ironically, branches blackenedd and tree died..Not from scale, but the chemical itself, since it died the day after applying insecticide.

I spray citrus with Fish Emulsion, in autumn, before bringing indoors..FE prevents and kills scale on contact.
If you find Fish Emulsion replusive, (it has a fishy smell that lasts about 2 days, but they now sell an odorless type) there are other oils such as Neem.

Artificial light will certainly help, but in your case, I doubt low light is/was the cause of your lemon's problems.

Stop fertilizing..For one thing, as you stated, light is low..Citrus go into a semi-dormancy..They need a rest period..If light is low, plants slow down, therefore, adding fertilizer can cause spindly growth. Also, when a plant is undergoing problems, disease/insects, it is advised to stop adding fertilizer.
If anything, in early fall, add a slow-release, low NPK type. In spring, or when growth is spotted, start your regular feeding program.

I disagree the Baby Shampoo did any damage. How much did you use, and what ratio of water?
One author and tv host, Jerry Baker, recommends cleaning foliage with baby shampoo..

Did you discard the Ficus? Considering it's a bonsai, scale shouldn't be too difficult ridding. For starters, you can hand-pick, unless you're squeamish about bugs..Matt, you're talking to a 'bugophobic,' lol, yet, if I saw scale on a plant, you'd best believe I'd start plucking, though spraying is preferable..lol

Prune blackened branches..they will not resort to green. Shake plant to remove dead leaves..Toss leaves in the trash, or burn..which reminds me..if you have other plants, especially neighboring, inspect each carefully..also, isolate plants with scale.
Move plants to a cool,(65-68F is fine) airy, humid location..and yes, humidifers help big time..fresh air is also important. If you have a ceiling fan, run on low at least 1 hour a day..it circulates air to a degree. I also use a rotating fan.

Matt, do you have a picture to post?

If your citrus is managable size, after removing dead leaves, haul to sink or shower, allow water wash leaves..2-4 mins. Hose Ficus too.

Matt, I'm an optimist when it comes to plants..only plants, but plants, nonetheless..Some of my plants were bought in what I coined, the 'dead cart' in stores where plants were going to be tossed..In and outdoor plants..I'd never buy an infested plant, no Sir, I'm not that optimistic..lol..

Good luck, Toni

If you have any questions, ask away..Please stop fertilizing, and before adding water, check soil, make sure it's dry before giving a drink.