Learn English – Authentic alternatives for “because”

The use of subjunctive mood ('could' instead of 'can') is probably due to the unlikeliness of "specific population" visiting that page. The use of 'can/could be' instead of 'is' most likely due to the same notion - possibility.

The problem does not exist unless we have evidence to support it. If there is no evidence but speculation, and the conditions are likely to lead to the problem, then the usual way to say is "can be". If the circumstances are questionable for the problem to arise, or predicated on some coincidence, then "could be" is advised.

Compare:

I have an uncle with a short temper who is 70. My uncle is an old grumpy man.

I have an aunt who is looking for a husband. With her looking, my uncle can be an old grumpy man.

My mother had a brother who died in his 20s. Had he lived, my uncle could be an old grumpy man.

As to your question about the possibility to improve, perhaps, but it is quite OK as it is. And it depends on what exactly you would like to say.

I think you're dismissing "once" too quickly. Once can mean more or less the same thing as "as soon as", depending on context. For example:

Children should begin learning a foreign language once they start school.

This implies they will learn the language from the beginning of the school year, not just anytime during the year.

Other phrases that you might use:

Children should begin learning a foreign language from the start of their first school year.

... right when they start school.

... upon starting school.

Somewhat weaker:

... as early as when they start school.

... around the time when they start school.