为什么but despite能够放一起使用；为什么but because能够放一起使用；but作为并列连词，应该放两个分句的中间，为什么这里放在了句首?

Use of English Ancient Greek philosopher Aristotle viewed laughter as “a bodily exercise precious to health”. But despite some claims to the contrary, laughing probably has little influence on physical fitness. Laughter does produce short-term changes in the function of the heart and its blood vessels boosting heart rate and oxygen consumption. But because hard laughter is difficult to sustain, a good laugh is unlikely to have measurable benefits the way, say, walking or jogging does.

1. but为并列连词，despite为介词，为什么but despite能够放一起使用，如果并列连词能与介词一起使用，那还有类似组合吗，从属连词是否也可以与介词一起组合使用；

2. because为从属连词，为什么but because能够放一起使用，并列连词可以和从属连词一起使用吗，如果可以，还有类似组合吗?

3. but作为并列连词，应该放两个分句的中间，为什么这里放在了句首。