LnD 2 or 3

Depends on who (what ivs) and what build you are using it for. Though the answer may well end up being 'yes'

I see Life and Death mostly used on brave builds and glass cannon builds. On those kinds of builds you want all the atk and speed you can get your hands on.

Every single point in speed is a bunch of heroes you can now double/quad.

For me, it would depend on my level of investment... If it's a unit that I plan on using long term (usually that means I like them visually and they are viable) then yes... If it's a unit that I plan on using for an event or until a better one comes along, then no way unless I'm willing to fodder them to inherit LnD3 to someone else (may not be an option if they have other useful skills).

Only if the unit is going to be one of your core Arena units in the long-term. Otherwise, 2 is sufficient.

IMO, Fury 3 is better than L&D in the majority of cases anyway.