What's your question? Ask

Oranges generally began to fall off the trees when they are ripe.
You can't tell by color, you can test the fruit to check on it's ripeness.

Lemons are among the most difficult to determine ripeness.
It can take 6 to 9 months for Lemons to ripen, depending on the conditions.
Warmth and plenty of water encourage the fruit to ripen as quickly as possible.
Lemons stay firm until they are overripe. Look for a slightly softened texture and fruit that is heavier then it appears. This indicate a fruit heavy with juice.
Because Lemons can produce flowers and fruit as long as the weather conditions are right, your tree may have fruit at various stages of ripeness.
The fruit you are referring too, may not be as heavy as past years.

Lemons are very cold sensitive, so whether you are able to maintain this crop of lemons will be up to Mother Nature.

Here are some links to help you.

https://aggie-horticulture.tamu.edu/citrus/lemons.htm

https://www.gardeningknowhow.com/edible/fruits/lemons/how-to-grow-a-lemon-tree.htm

In reference to the 'the tree growing back'.

I do think that you actually have a tree that grew back from the grafted root stock of the original tree that you removed.
The lemons that are now being produced are not likely the same fruit as you originally had on the tree.
Here is a link that explains grafting.

https://www.gardeningknowhow.com/ornamental/trees/tgen/can-grafted-trees-revert-to-their-root-stock.htm

Here is a link that will help you.

https://www.gardeningknowhow.com/edible/fruits/lemons/lemon-tree-pruning.htm

Because a Lemon Tree will bloom all year long, you can plant them any time of year.
Water new trees throughly and watch the moisture levels carefully the first season.
Apply 2 to 3" of hardwood mulch to help with moisture retention.

https://www.gardeningknowhow.com/edible/fruits/lemons/how-to-grow-a-lemon-tree.htm

Here is a link that will help you.

https://www.gardeningknowhow.com/edible/fruits/lemons/harvesting-lemons.htm

Black Citrus Aphids are tiny little soft bodies insects that that feed on the juices of the young citrus leaves. Their feeding cause the leaves to turn inward toward the stems. They rarely feed on the older leaves so control may not be warranted. Many growers leave the Aphids alone, as they produce honeydew that other pests eat, thus keeping them off of the fruit.
Here are some links with more information.

https://www.gardeningknowhow.com/edible/fruits/citrus/curling-citrus-leaves.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm
https://www.gardeningknowhow.com/edible/fruits/lemons/pests-of-lemon-trees.htm