这句英语反意疑问句对不对？

对
也可以说
We have plenty of time to get it done, don't we?

这是正确的，不过比较正式的是
We have plenty of time to get it done, DON"T we?

应该是isn't it吧。

错了，因为have 之下这里是有的意思。
我不清楚你最后问的是“是吗？”，还是“难道不是吗？”
答案应该是We have plenty of time to get it done, don't we?
或者We have plenty of time to get it done, do we?

对
We have plenty of time t get it done don't we?

应该是We have plenty of time t get it done don't we?
因为这里的have是"有"的意思,而不是情态动词,所以不能用haven't we.