Hi @Coop
To answer your questions:
1. You describe two different situations. If a non-EU national is living in another EU country and has a job offer in Germany, then they must apply for a work visa from the German embassy in that country.
If the person is in Germany on a Schengen visa as a visitor and finds a job, it is very difficult to transfer from a Schengen visa to a work permit. It is a decision that is at the discretion of the immigration authorities (Ausländerbehörde). It is only possible in cases where the possibility of a job and the wish to stay in Germany to work came after the person comes to Germany. The person will have to convince the authorities that they were genuinely in German as a visitor (i.e. not coming to **** for work) and the job opportunity just presented itself. Otherwise, the authorities will assume the person is using a Schengen visa to get around the more lengthy process of first getting the appropriate work visa for entering Germany.
2. There are jobs available in Germany without knowledge of German. Aside from the IT sector, the main ones which come to mind are teaching in English at an international school or university or working in a major international company.
3. While on the Opportunity Card, a person can work part-time (up to 20 hours a week) in any kind of job. They stay on the Opportunity Card. Only if they find a skilled job can they change to a work permit.
4. In the example you give, I do not see what the relevance of the new rules are. The person can apply for a work permit or an opportunity card at the German embassy in Latvia. If they come without such a visa, the situation from my answer to question 1 applies.
Best,
Éanna