您好，请帮看下以下问题：

在赵俊英的现代英语语法17.4.3双重定语从句这里，第二种情况，两个定语从句在同一层次上，有一些例句：

1,The house he bought in 1986, and which he sold two years later, is again on the market.

请问是否可以改成The house (which) he bought in 1986 and sold two years later is again on the market.

这种情况括号内的which是否还可以省略？

2,Peter shared a flat with a boy who was a philosophy student and who liked to sit up half the night discussing philosophy.

是否可以改成Peter shared a flat with a boy who was a philosophy student and liked to sit up half the night discussing philosophy.

这个句子改了以后可能会产生歧义，因为liked的主语可能被理解为Peter。如果是因为这个原因不能改的话，那么把a boy换成确定的his best friend, 请看下面这句是否正确？Peter shared a flat with his best friend,  who was a philosophy student and liked to sit up half the night discussing philosophy.

3, This course is intended for students who already have a good command of the basic grammar of English and are familiar with a reasonably wide selection of vocabulary, but who wish to increase their competence in the language.

这个句子的第一个定语从句有两个并列的谓语(have和are familiar)，假如把but后面的内容全部去掉，but前面改成句号，应该也是一个成立的句子吧。

那么能否把but保留，然后后面的who去掉，让wish和前面的have，以及are familiar组成三个谓语的并列？

最后引申出自己写的一个句子，Ecotourism is a healthy way of travelling, which can not only get people close to nature, but also raise their awareness of environmental protection.定语从句是一个简单句，用not only和but also 并列了两个谓语，这样是没问题的吧？