你的问题有两点需要注意：

1. 倒装语的虚拟语属于比较正式的用法，在口语或非正式文体中用得较少。

2. 倒装语的虚拟语气放在主句前面或后面都可以的（尤其是当倒装的结构比较长时），所以你的句子也可以写成：

(1) He would still not excuse their action were it all true. 即使这都是事实，他也不会原谅他们的行动。

(2) No one would blame you should you change your mind. 万一你改变主意，谁也不会怨你。

(3) I would have written before had I known your address. 我要是知道你的地址我早就给你写信了。

更多相关例句：

(1) You’d be surprised were I to tell you how often that question arises. 假如我告诉你那问题发生得多么频繁，你会感到吃惊的。

(2) You’re entitled to a full refund should you be unsatisfied with the quality of our goods. 如果对我们的商品质量不满意，我们可以全额退款。

(3) He mused on how different his life would have been, had he not met Louisa. 他默默地想，如果没有遇到路易莎他的人生会多么不同。