Have you heard that there are two types of causative form (long and shortened) in Japanese? What you are seeing is the 連用形 (aka masu-stem/pre-masu-form) of ずらせる/ぬがせる (ichidan), not ずらす/ぬがす (godan). The masu-stem of ずらせる happens to be the same as the imperative form of ずらす. (I assume you know how 中止法 works, but in case you don't, please see the links in the comment section first.)
The problem is, while ずらせる should be technically correct as the causative form of ずる, modern Japanese speakers almost always prefer ずらす over ずらせる. Basically you can think ずらす is an established transitive verb rather than a form of an intransitive verb. Similar verbs include 動かす; while 動かせる should be technically correct as the causative form of 動く, we almost always use 動かす (動かせる is normally regarded as the potential form of 動かす). Likewise, ずらせ in your example is technically not wrong but sounds fairly unnatural to me. (I don't know whether this is Murakami's style.)
On the other hand, there are many verbs where long and shortened causative verbs are used interchangeably. Example include 泣く (泣かして = 泣かせて), 驚く (驚かせ! = 驚かせろ!) and 巡る (巡らした = 巡らせた). 脱ぐ is one of such verbs, and ぬがし, ぬがして, ぬがせ and ぬがせて sound equally natural in this context to me.