Even in Cat 3 conditions they seem to land 27L and 27R now did they used to do that ?
Why not? 27L/R both have Cat III ILS and have had for many, many years.
Airlines seem able to change inbounds approaches in times of delay ie a BA 319 last winter heard via the scanner gave his slot up to a BA 747 lower on fuel.
Again, BA have been doing that for a very long time. (And you should a) not be listening on a scanner and b) you must not publish anything you hear - two offences under the WT Act).
Similarly a BA 77W seemed able to take the slot of a BA 321 well infront as he stated he didn't have much fuel.
See previous answer. Bottom line is that it is up to the captain of the a/c which will pick up the delay to agree. This procedure does not affect any other flights, just the two participating.
A decade ago if there was fog a plane could either hold or had to divert.
What else, may I ask, would be the alternative? They hold but if the delay is too long they go elsewhere. Has been like that for 50 years or more.