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This is the natural process when the light gets reduced. They will go into a semi dormant state. The only way to prevent this will be to provide at least 200 watts of horticultural lighting for the plant. There are very many options available, but I am particular to LED lighting.

As long as you purchase a good LED from a reputable company, then it will be of a good quality, and will actually save on your energy bill while being much cooler to run. I use a brand called KIND LED myself, but these are top end, and will run quite expensive. There are many options available, but make sure to do the research when finding a good quality light for your citrus.

I would expect a nutritional deficiency. Do you have resources for soil testing and prescription mineral amendments based on actual deficiencies found?

Deep watering is important.

Affordable soil fertility building can be achieved with legume green manure inter cropping.

Mulching helps retain soil moisture content.

I don't know local conditions and citrus problems in the Bahawalpur region, I am located in Southeast Asia.

You may want to seek authoritative help from a local agricultural university extension program, or local crop advisor. Or seek friendly exchange with other local growers.

If it is actually spiders, then they are not plant pests but in fact pest predators, beneficial members of the ecosystem that will help to keep real pests in check.
If they are spider mites, a true plant pest, you won't normally see them without a magnifying glass, but excessive fine webbing can be noticed.

In either case, if you consider the spiders a nuisance or a negative aesthetic issue, of if in fact you need to control a spider mite infestation, you can use an insecticide-miticide product like this:
https://www.domyown.com/foxfarm-bush-doctor-force-of-nature-miticide-p-17475.html

That would do as much damage as leaving them inside with little light. There is a very easy remedy for this, as citrus are not very picky about types of light. Any horticultural grade light of 200 watts or above will suffice for a citrus over winter. Using LEDs will cut down on lighting cost, and heat.

Using lighting indoors will often lead to fruit, where as the shock of being indoors without light in the winter will not allow for fruiting, since they will lose the majority, if not all, of their leaves.

I don't put my citrus out, even in the summer. I find indoor lighting sufficient. This is especially true in indoor greenhouses commonly known as tents. This provides an enclosed environment for trees and plants as long as you have a way to evacuate air.

If the tree does not have a slow period during the year, then it would be safe to prune this at any time. Make ure to keep any "sucker" limbs from growing at the base of the tree as well. Since your tree produces so well, I wouldn't be worried about hurting it. Just make sure not to take off more than a third of the tree at one time. If you have to do major trimming, I would suggest doing it over the course of a few days instead of all at once.

When you grow from home-collected seed, the results are uneven. Most fruit is grown on hybrid trees and flowers may be cross-pollinated with a different species. All this means the genetics of your seed are all over the map. And some seed may be sterile. In the fruit trade, they avoid this problem by grafting the desired hybrid to a vigorously growing base. They do not grow from seed.