The EHP6 for ERP6.0 is currently in the ramp-up phase and still not released for general availability. For more information please refer the below SAP link which provides more information.
According to this,the ramup release date for EHP6 for ERP6.0 is
The general availability for EHP6 for ERP6.0 is May 2012.
Please note that the above dates are planned dates and are subject to
By the way, note 1409196 shows how to join the ramp-up product.
Only the Rampup customers can access the data related to the EHP6. For more information about the EHP6, please refer the note specified below.
1566412 - Enhancement package 6 for SAP ERP 6.0: Required SWC
I believe you've get clear answer for your question 1.
2. Is compulsory that system should be included in SLD ?
>>> for SOLMAN 7.1, yes, this is mandatory. Since solman 7.1, SLD is sync with LMDB, and changes in LMDB will write to SMSY automatically. You can't find your instance in SMSY if it's not included in SLD.
for SOLMAN version lower than 7.1, you should able to send the data to SMSY via RZ70, but this only applies to ABAP stack, for JAVA, yes, you still required SLD.
3. Landscape verification Is compulsory ?
LV is not compulsory, but is good to have. LV helps to detect inconsisteny on your system landscape, eg: system without product system assigned, wrong product version and etc.
Hope it helps,