Is it permissible to rebuke young blacksmiths who do not perform Friday Prayer, given the doubt as to whether they are excused, and does the doubt of potential harm to their work prevent rebuking them?

The basic principle is that attending Friday prayer is obligatory for every sane, adult Muslim who has no valid excuse. You can advise these individuals and explain the obligation of Friday prayer for those who are not excused, and that deliberately missing Friday prayer is among the major sins. If they have a religiously acceptable excuse, then they are excused. Otherwise, you have absolved yourself of responsibility. Your doubt should not prevent you from advising them gently and wisely.