I guess essentially, if the channel strip has been reset so has no synth plug-in and i select a lead sound from the library
"A lead sound" - is this a plugin setting, or a channel strip setting?
the highlighted channel strip will then have the relevant synth (say exs24) plugged in that I have selected the lead sound from.
Ok, so it sounds like you are talking about channel strip settings.
That means i have plugged in the exs24, but not directly, rather by selecting a lead sound from the library that just happens to be from the exs 24.
No, you have loaded a channel strip setting that loads the EXS24, sampler instrument, and various plugin settings.
Why do only some of all the available synth leads from all bundled soft synths appear under the Synthesizers>Lead headings in the library?
Because these are channel strip presets, not plugin settings. Two different things. Some channel strip presets are made using an instrument, and instrument presets, and various plugin parameters, but they did not make multiple channel strip presets for every plugin setting.
If only some synth leads are here at random, can I customise the list so all the synth leads from all the various synths appear here?
If you want to save every plugin setting of every instrument as a channel strip setting, you are welcome to do that, but it'll take months of very tedious work for now real benefit.
Remember, plugin settings can be flipped through very quickly as there is no plugin changing to be done, whereas channel strip settings load/unload multiple plugins and are much slower.
Besides, they are two different mechanisms - if I'm making ES2 synth patches, I want to be able to save those synth patches (plugin settings) and if I want to also include other plugins (reverb, delay etc) then I'd save the patch and all plugins settings as a channel strip setting.
Bascally, it sounds like you are looking at channel strip settings and wanting plugin settings to appear in the same list - and I'd argue that would be a very bad idea, as they are two different things.
However, it is up to you to organise your library how you want. I'm not saying the idea of having everything available easily from one place is a bad idea, it's not, but the current scheme in Logic doesn't quite work like that.
So this is a channel strip settings vs plugin settings thing, unless I'm just not understanding you...?