1) Put in the information that is needed to make the purpose of the method clear. You can probably halve the length of your example name without any loss of understanding about what it fits.
2) guidelines are guidelines. Don't slavishly follow rules when they become counter productive. If using an abbreviation makes it easier to read and understand the code, use abbreviations. The main thing is to try to limit abbreviations to long names that are commonly used, and use intuitive and commonly used abbreviations for them, so that anyone reading your code can easily work out what they mean. For example, decl is a common abbreviation for declaration, and difficult to mistake for anything else.
3) sometimes you can avoid the need to abbreviate by using a synonym.
I think you could probably drop interface and member from your name without losing the meaning. But perhaps the "explicit interface implementation name" is actually the "explicit name" - explicit has a well defined meaning, especially in the context if your class, and you can always add the fully watertight legal definition in your documentation comment. So: "GetExplicitName"