How to make lemon and chilli liquid to have with hummus

I don't think this is practically possible with lemon juice, as milk protein will curdle in the presence of acid (in this case, citric acid in the lemon juice)—and the process happens much more quickly at hotter temperatures, as in tea.

A pinch of salt may slow the process, but is likely to be unpalatable.

You could neutralize the acid with baking powder, but getting the right balance would be difficult, and again, the result is likely to be disconcerting and unpalatable.

If it is not the acidic balance that you appreciate in your tea, but rather the flavor of the lemon, you might try making a simple syrup with lemon zest, and using that to sweeten your tea. It will give the lemon flavor, without the acidity, and so not trigger the curdling.

Lemon curd is not cooked so much for a time—in general times are only guidelines to help cooks not yet familiar with a recipe do planning—as they are to a specific outcome.

The traditional test for lemon curd (and all custards, really) is the nappe, or coating the back of a spoon. If you dip a spoon into the curd, and then run your finger through it, the little "mound" or "ridge" of curd that builds up on either side should not immediately flow back into the path of your finger. The clear path should remain. This tells you the curd is thickened.

(Image credit: Baking Vintage)

A more modern method would be to take the temperature with a high quality instant read thermometer (the curd should be well stirred, so the temperature is a good representation). It should be approximately 180 F / 82 C when fully thickened.

The amount of time it takes to get to this stage depends on your volume and your heating method.

I have never heard of a lemon curd being supplemented with additional pectin (there may be some natural pectin from lemon zest already in the recipe); the thickening agent is the proteins in the eggs, which is why it is a custard variant.