OK guys, I don't post very often, but I am going to put myself out there because I am a citrus grower and I have a small grove with around 30 trees. I have had citrus in my yard for over 20 years and I hope I can help you out as best I can.
I don't grow indoors because I live in Florida, however I think it should be noted that I am in zone 8 and it gets cold here in the winter, sometimes it gets below 32, but it doesn't last. The ground doesn't freeze. If it got below 32 and stayed that way and didn't wram up in the day and we got a second night below freezing, not only will the plant lose the fruit it had, but it's not going to set fruit anymore (very rarely that it will), if it hard freezes, there is a chance the plants will die. A secret of citrus growers in my area is that we HOPE for a night or two of under 32 degrees where we get "kissed" by the frost after the fruit has set. If you can get those ideal conditions the fruit will be considerably sweeter.
Anyway whatever way you are tempering your plants to take them back out, do it the same as you did last season. While I said that they can take the cold, they are not going to like it if they haven't experienced it before, so just keep going with the same temperature limits that you had last season.
Next... the plants in these pictures are small.....really small.... I would never let a plant that size set fruit myself. I do have the luxury of growing right in the ground, but.. if you have cuttings and they start setting fruit quickly, I strongly recommend pulling the fruit off as soon as it sets and not letting that plant produce fruit for the first 2-3 years (or even the first 5 years) and keep them in the vegetative state of growth so they can get strong and bigger than that to support the fruit. It is possible to get branches of fruit that hang down and will pop back up after harvest but it's not good and doesn't produce that full, meaty, high-quality fruit that you are looking for. By keeping the fruit off it for the first few years, you will have STRONG branches that are capable of bearing GREAT fruit. When the plants is limp holding all that fruit, it is a sign that your plant is not capable of supporting the fruit and could suffer great losses (not just fruit, but death of the plant) if you have a condition with strong winds, or even worse, a pest infestation while you have all that fruit on the tree, you will suffer a great loss.
My next bit of advice scares me when I read it because you are growing in pots, I am not sure if this can be done this way because I grow in the ground. I am going to make a call to a close friend tomorrow to ask him about it, because I am not 100% sure about this next piece of advice. You may need to spread your fertilization out more than 4 times, like 6 r 8 just to be careful about not burning the plant.
Next, when you get to the point that your trees are strong enough to bear fruit, the real secret, above all else is fertilization. A mature Myers can be fertilized with up a total of 3lbs of nitrogen per year spread out over four applications. (The lemons trees in this thread are not mature trees by any stretch of the imagination). The nitrogen is the true secret in setting fruit in citrus. Other varieties of citrus can go all the way up to 5lbs of nitrogen over the course of a year, but Myers lemon requires less. Around 3lbs is the limit, I usually use 2 - 2.5 lbs. This, and pollination are your secret. Since you are indoors you will have to hand pollinate, I have never hand pollinated these, so you have to take the advice of others on that. Outdoors, I rely on wind, and bees. I have a hive on my property, but most experienced citrus growers that have several trees don't have hives and they still do just fine (outdoor).
If you follow my advice, and keep the plant from freezing and properly fertilized, you will be absolutely amazed when you see this plant blooming year round. That's when you know you have it "just right" and you can have lemons all year from this.
Lastly, you shouldn't prune these trees. They are self pruning, what happens in the wild (and on my grove) is that a branch will die when it's ready, it will dry out, and the wind will knock it down (or I'll come remove it by hand at that point). I never prune mine unless the branches are touching the floor (which doesn't always happen with Myers). Some people will top the trees to keep them at a reasonable size so they don't have to pull out a ladder to pick the fruit from the top, but I just use a pole with a little clipper and basket to catch the fruit as it's cut. Don't let them fall, because it will bruise the fruit. If you pick at peak ripeness the fruit will most likely break when it hits the ground depending on it's weight. In my experience the Myers doesn't really canopy like, say a grapefruit would. It is usually pretty erratic in it's growth behaviors. The only time you should ever prune this tree is if the root stock starts shooting off branches, you can cut those if desired so you don't have a "fruit salad" tree. But it's not really necessary, and heck who doesn't like extra fruit!?
some other notes:
you want really great draining soil, they don't like wet feet at all... make sure your mix drains really well....
another tip is to get your soil analyzed once a year (I don't always do this but I never go longer than 2 years without a proper analysis), once you have an idea of what kind of nutrients exist in your soil, you can then decide how much fertilizer you want to add. If you over fertilize your trees, you can burn them pretty quickly. I don't always put 3lbs every year, sometimes I put less if I analyzed and see that I have a lot of existing nitrogen in my soil from previous applications. This is pretty important especially when you are dealing with small plants like the ones pictured, and you are planting in pots.
I usually use a "citrus blend" which has all the trace elements and micro-nutrients required by Citrus. A general purpose fertilizer simply will not do. I then supplement this fertilizer with additional nitrogen. I typically will use Ammonia Nitrate that I side-band around the tree along the drip line, but you can use an organic source of nitrogen as well. This is my secret to setting TONS of fruit each season. I don't know how to go about this in a different country, but in the United States a local extension office will test your soil for free, and often will help you calculate exactly what to use to get your desired results.
disclaimer: I am not an expert on this, but I have done a lot of research at the University of Florida who is one of the leaders in Citrus Research in the world. I have talked with several professors and other veteran growers on the topic and I have had great success growing citrus. I really hope that I have helped shed some light on this subject and I can give back to the forum that has given me so much!