Whenever amatsukaze_(kantai_collection) is seen without her serafuku (such as post #1669151 and post #1673876), are the garter_straps still considered garter straps that are stretched over her top body, suspenders since it is also considered holding up the stockings, sling_bikini as it shares the description or something else?

Well the garter_strips tag was created to cover various straps that hold up thighhighs/stockings that were not garter_belts (or at least no garter_belt the object was visible to tag), so by that definition those images would qualify as garter straps.

Not sure on calling them suspenders, don't those normally loop over the shoulders? If it does qualify, I don't see an issue in being able to label it both garter_straps and suspenders then.

I wouldn't label them sling_bikini since it is not a swimsuit and doesn't seem to be a sling bikini. The straps don't hold up the crotch piece at minimum, they're going under the crotch piece and connecting to thighhighs.

In British English, suspenders = the straps of a garter belt. Braces = over-the-shoulder suspenders. I think that the British meaning was being used. Then again, hers do go over the shoulders, so maybe I'm off topic.