It sounds like you're finding your way, and as pointed out by the prior post your initial confusion likely stems from similar naming between Preset banks and Sample groups. There are parallels between them: Factory (F) Presets and Samples come with the device and are only updated by Sequential, Add-On (A) Presets and Samples only come from 8dio, and User (U) Presets and Samples are freely available for your use as well as for third-party content. The sample import process can theoretically be used with any of the three, but I don't think there has ever been a factory sample update.
Once imported, sample "instruments" are found in a combination of group + type (sometimes called category) + sample number, which must be unique. So you can't have two U1-02.Bass-#2 instruments installed at the same time, but there are lots of unique slots. 32 user groups, each with 17 categories, each of which can have 98 instruments (numbered 2 through 99, as 1 is always reserved for silence.) You're much more likely to run out of the 50GB of available sample storage than unique places to store them, unless you're using a lot of older samples that are tiny by modern standards.
You can indeed see what's installed, how much space it takes up, and can delete samples you've imported. Press Global, then scroll down to function #32, Manage Samples, and select it. You can see each of your installed user and add-on groups here with an indication of their size, and can delete entire groups. There is no way I'm aware of to delete individual sampled instruments.
Lastly, I don't believe Sequential ever officially documented the procedure for packaging presets along with samples. 8dio does so for their add-ons, but third parties tend to provide preset banks separately from their samples. This is generally necessary as there's not a lot of standardization regarding what presets go with which samples, and there are far fewer user preset slots than user sample locations. 8dio appears to have limited themselves to 32 presets per add-on for similar reasons.