1. however和whereas

You can't use 'however' and 'whereas' in the same way. Follow these general rules in order to avoid grammar mistakes:
However和whereas的用法不同。请遵守下面的规则,以避免错误。

Use 'however' (followed by a comma) at the beginning of a sentence to contrast with the previous sentence:
在句首使用however(后面要加逗号)来表示和前一句意思相反。

Some people believe that schools are responsible for the behaviour of their students. However, others argue that discipline is the responsibility of parents.

Use 'whereas' (after a comma) to contrast two ideas in the same sentence:
Whereas用于同一句内的两个相反意见(前面要加逗号)。

Some people believe that schools are responsible for the behaviour of their students, whereas others argue that discipline is the responsibility of parents.

You can use 'on the other hand' or 'by contrast' in the same way as 'however'. You can use 'while' instead of 'whereas'.
On the other hand和by contrast的用法和however相同,而while和whereas的用法一样。

2. while, whilst, whereas

While and whilst are the same, but whilst is a bit more formal (according to one famous grammar book).
While和whilst一样,但whilst要更正式些(一本著名的语法书这么说的)。

Whereas is always used for contrast. While/whilst can be used for contrast, or to mean "at the same time".
Whereas总是用于表示相反的意见。While和whilst既可以表示意思相反,也可以表示“与此同时”。

As can mean "because" or "exactly at the moment when".
As可以表示“因为”,也可以指“就在那时”。