Baking – add butter to yeast dough which has already risen

As it turns out, my dough was very cold. Due dilligence: here's a full answer.

One of the requirements of yeast for fermentation is appropriate temperature. Cold dough straight from the fridge won't rise, or it will only rise very slowly. 25-30°C is a recommended temperature range for rising, although cooler temperatures can work.

Duh.

You cannot keep the texture if you are using that much sugar.

Two tablespoons of sugar per cup of flour is the maximum that you can add without major gluten damage. [...] Too much sugar is also damaging to the yeast.

(quotation from Cookwise by S. Corriher).

She goes on to explain that certain kinds of bread are made with more sweetness (including the Portuguese sweet bread rfusca mentioned), but they use a combination of many different sugars and malts. It is possible that your bread may become edible texture wise if you cut the amount of total sugar by half and then experiment for a few dozens of batches with different classes of sugar to get a workable texture. But I doubt that it will be worth it.

When you bake, don't think of sugar as a sweetener. Baking recipes are always calculated such that the sugar amount is right for the correct texture, not for sweetness. If you want sweet bread, use an existing recipe for a brioche style bread, which is somewhat sweetened. Or if you want it extra sweet, use a recipe for very sweet bread, but be prepared for it to handle a bit weirdly during kneading and to end up with a cakelike texture. But if your brother wants to eat something very sweet, don't bake a bread, bake a cake.