在月下花前，一双对鸳鸯谈个恋爱诉衷情。

1. We use "双" for shoes, socks, chopsticks, eyes, etc. "一双鸳鸯" is rare, normally seen in a song when the pronunciation is preferred. Sometimes you may even find "鸳鸯一双" at the end of a sentence to meet the rhyme. For a couple of animals (male and female), we usually use "对"，for example, "一对螃蟹", "一对石狮", "一对老夫妻", and so on.
2. Chinese is concise, so it's better to leave out "在" without changing the meaning. I like the word order you choose to put "月下" in front of "花前" to break the cliché.
3. "谈个恋爱" is totally fine, just doesn't fit in with the context. "谈恋爱" means a series of things that happen over a period of time. Here we only have one scene.

飘风一直刮疾风肆虐着时光的脸色容颜，但我会等你一辈子。

如果感情没变，在若你不曾改变，那么来我们最喜欢爱的那棵樱花树下见我。

"感情" can be omitted when we all know what's not changed. In this way, you will feel more reserved.

天晚了，鸳鸯早就已飞走了-他们的羽毛掉到——羽毛飘落池塘。

Again, Chinese is concise. I've changed some words to make it more like poetry.

时间慢慢的溜走，每一分钟想一滴一滴的雨如同一滴雨珠的落下，

每一秒想花瓣落到地上彷佛一片花瓣的下落。

残酷无情的士兵奔驰地驾驭他们的骏马策马飞驰而去。

他们只想心系战场，不睬见自然。

The original sentence is completely okay, but if you want to be more like poetry, you have to use more formal words.

众星/群星/繁星座连着天空，簇拥/围绕着山顶。

"星座" is only used in astronomical and astrologic context. You can use "众星" to personify the stars, collocating with "簇拥". If you don't want that implication, the two "群星" and "繁星" will be almost the same, and "围绕" stays fine.

这是仙人的走步道。

走道=过道，means aisle in a building.

仙人下山，苍凡人上山。

There is no such word as "苍人" in Chinese.

在月下花前，一双鸳鸯谈个恋爱在缠绵。

动物一般不说「谈恋爱」...

飘风一直刮时光的脸色，但我会等你一辈子。

狂风摧毁了岁月的脸庞，但我会终生等待你。

如果你对我的感情没变，那就在我们最喜欢的那棵樱花树下见我。

如果你依然对我怀有情愫，那就在我们最钟爱的樱花树下同我再会。

晚了，鸳鸯早就飞走了----他们的羽毛掉到落在池塘。

时间已晚，鸳鸯已不见身影，唯有散落在池塘的羽毛。

时间慢慢的溜走，每一分钟想一滴一滴的雨，都像雨水般、

慢慢**地**溜走

每一秒想花瓣落到地上都像飘落的花瓣般有去无回。

残酷的士兵奔驰地驾驭他们的驱驰着骏马。

星座连着天空，围绕着山顶群点缀成线，在山顶盘旋。

仙人下山，苍人上山。

我不知道你在哪里看到的「苍人」，我完全没见过这个词。