I would like to know the following:

1) For passenger who has no lever with airlines ( SQ and UA) and book business ticket, the first segment is SQ flight, and the second segment is UA flight. Does he / she has an access to use SQ business lounge at Changi airport . If not, which lounge she / he can gain an access to .

2) Which airline to credit the miles earned to get the most of it .

Please advise and thank you .