Re 3.
/e/OS builds on the LineageOS code, but modifies it considerably. The e-development team has proven several times that they can support devices for much longer, even after Lineage stopped supporting them. However, this is highly dependent on the availability of manpower (“we do have developer bandwidth issues”).
Re 4.
Currently there is no official /e/OS ‘T’ / 20.0 aka Android 13. /e/OS 1.14 will ship with ‘S’ / 19.1 aka Android 12 at most.
Until the planned delivery of /e/OS ‘T’ in 9/2023, you can gain first experiences with custom ROMs by installing LineageOS 20.0 for MicroG, for example. The change to /e/OS ‘T’ V1.15 will then be very easy in September 2023.