Ok, again, several questions there..
1] Some "sounds" in the Library are not instruments that you can play, they're just processors that you can apply to a live external signal [like a real guitar or your voice]. So if you want to play a guitar sound and you don't have an external guitar, you need to create a Software Instrument track, not an Audio track, and choose a *virtual* guitar Instrument, which you can then play [& record] from your MIDI controller [keyboard]. Then you can add processing plug-ins [like Amp Designer] to that track, and call up presets [like Awesome Tweed] for those processing plug-ins.
2] If you don't have a real guitar, then Input Monitoring is irrelevant, because there's no external Input to monitor -- again, if you don't have a real [external] instrument to plug in to GB, then you'd need to create a Software Instrument track, *not* an Audio track.
3] Yes, that's correct, Flex is only for real instrument [Audio] tracks. Software Instrument tracks automatically flex [follow the tempo] so there's no need to have a Flex feature for them. Flex makes your Audio tracks behave the same way your Software Instrument (MIDI) tracks already behave.
4] You mentioned those presets [Clean guitar, Amazing Tweed] specifically in your original posts, so they must be in there. Did you create a Software Instrument track and select it as shown in the screenshot [to access the Classic Clean instrument preset in the Library at the left], and open up an Amp Designer plug-in window from the Smart Controls pane [at the bottom] to access the Awesome Tweed preset [in the preset menu within the plug-in]?