Q1:To be done 15 to 45 min prior to CTA boundary.
Q2: HF SELCAL watch is still required. Shanwick will still give you HF freq in case of logon problem. In case of logon problem, you have to return to voice.
You may use SATCOM (if equipped) in case of COMM FAILURE.
Q3: UK departure are to request clearance as soon as possible after departure (UK AIP ENR 2.2). If using ACARS, you MAY request the clearance via datalink, for entry points on the Shanwick eastern boundary (from ATSIX to PASAS) without the requirement to contact Shanwick on RTF.
This system should not be used by aircraft within 30 min of the Shanwick boundary; VHF or HF should be used.
You can find those information on the Jeppesen (if used) Atlantic Orientation charts AT(H/L) 1 and 2.
Q4: Refer to your SOPs
What VinRouge gave you is an excellent reference website. The FAA has one also for the Pacific.
Happy crossing.