What is the primary distinction in the appropriate usage between the grammatical markers '-는 셈이다' and '-는 모양이다' when expressing an inference?

The primary distinction lies in the level of certainty and the basis of the inference. '-는 셈이다' expresses a logical conclusion or a strong inference based on clear, objective evidence or calculation; it implies a high degree of certainty that something is the case. For example, '세 시에 출발했으니까 지금쯤 도착하는 셈이다' (Since....