Much time has passed since the eggs were laid and I thought I got away with it and the three real ones I left in the nest were not going to hatch.
They did, at least one did because I could hear the baby this afternoon. I haven't been able to see it because Frank is being uncooperative in a way he wasn't the last time. I tried to move him aside and he was having a fit, so to avoid him trampling the baby I had to leave him alone.
I will go back to it tomorrow when they are out in search of food.

My question is: When can I take the fake eggs away?
I am afraid that they can hurt the baby, but I want to avoid Trilly panicking and laying new ones to replenish the nest.

How many eggs/babies are in the nest total? If it's six or less, you can leave them all in the nest for now. The fake eggs aren't a danger to the baby (unless they're significantly heavier than a real egg), and they'll help retain the heat when the parents are out of the nest. Unhatched eggs give the babies something to lean on, so it's not bad to have them in there for a while longer. When the oldest baby is about a week old you could gradually remove the fakes.

There are 7 in total (this is the number she lays usually), 3 real and 4 fake eggs. The fakes are not much heavier than the normal ones, just slightly bigger.

If you get an opportunity, you might want to remove one of the fakes, just to make sure that mom and dad can cover everything with their body surface. We wouldn't want the baby to get cold because there wasn't room for it under the parent. The parents should be able to handle six eggs/babies pretty easily because this is a normal clutch size.

Ok thank you tomorrow I am going to remove one. To be honest I hope to find that the other real eggs aren't good so that I can remove those.
You know that picture I posted of Enormous Frank? Well he had all the eggs underneath, I have no idea how he does that, he really spreads himself lol.

It's probably perfectly safe to leave them all in there but this is just a little extra precaution. The eggs can't wriggle out from under him but a baby can, and a little extra space underneath Enormous Frank will make it less likely that this will happen.

Is there any way I can avoid him to get stressed and thin as he was with the last clutch? He was really pathetic, poor boy, he even gave me a scare when for two days in a row he had VERY smelly morning poo, I swear I never encountered a smell like that before in my life.
I was about to take him to the vet but after two days the smell was gone (in the new poo I mean).
I am putting Guardian Angel in the water, sprinkling brewers yeast on food, giving him as much variety of food as I can, but maybe there's something else I can do to sustain him.
I caught Trilly licking the butter plate (not him though), maybe it's a sign that they lack something?

Butter is just plain delicious and I would lick the plate to get it too. But if it was salted butter she might be craving salt. Breeding birds need more sodium in their diet than nonbreeding birds do, and sodium deficiency can lead to parent birds plucking the babies. Chard (silverbeet) is an excellent high-sodium vegetable that contains a lot of lovely nutrients.

Believe it or not, Chard is not to be found in this nation of veggie haters!
I give them celery which I heard contains sodium, is that correct? They don't particularly like it though.

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Yes, celery is high in sodium but it doesn't have a lot of other nutrients. If they don't like it, there are other ways to provide sodium. A small number of salted sunflower seeds each day, for example. A small amount of salted butter spread on a chunk of whole-grain bread. Or hang up one of those salt wheels made for rodents.