I meet and exceed all the requirements for DEC except the JCAB ATP(L). Are they hard core about this or do they consider waivers (even though they advertise they don't)?
The only DECs in recent times have held both a JCAB ATPL and 767 type rating. Read into that what you will but the contract agencies will probably have a better idea of the true state of affairs.
Also, if I were to join as an FO, do they do accelerated command upgrades?
Nope. You wait your turn and then rely on your previous captaincy experience to ensure you pass the assessment and upgrade.