First off all, I edited your post a bit. I removed the link to the guide of the other site, because that is against the rule here. I also removed the link to amazon as it might come across as advertising.
I will give a more elaborate answer to your questions, so you actually understand how things work. In the wii, almost everything is run using IOS. IOS are used to run games and also the system menu, just about everything really. When you launch a wii game, the game will specifiy which IOS it will use. The IOS is launched and after that the game is started. There are multiple IOS which can co-exist, each installed in a different so called slot. For example, mario kart uses the IOS located in slot 36 and hence it's called IOS36.
Every IOS has different features. For example, IOS58 is the only IOS which includes native USB 2.0 support and is used by the HBC. They could have used a different IOS for the HBC, but then there would be no USB 2.0 support, or an IOS would need to be modified to include USB 2.0 support and hackmii (makers of HBC) are not fan of modifying IOS'.
One problem with these IOS is that they contain code that checks whether the game is an original disc, something that is undesirable for back-up loading. So in order to load back-ups, custom, or cIOS were made. cIOS do not contain the code used to check the disc anymore and furthermore they also include some extra features.
In the past, there were two main creators of cIOS: Hermes and Waninkoko. Hermes was the creator of IOS236, whilst Waninkoko was the creator of IOS249. Eventually, Waninkoko stopped working on his cIOS. It was then picked up by the d2x team, which continued to develop it.
So yes, you need d2x if you want to usb load. And yes, it is already included in the softmod any wii guide, as is IOS236 (which is not required for installing d2x at all). As to your final question: it is possible to remove an IOS, but most people just overwrite them with new ones.