I think the take over of Jubilee and their TL11 engined Lynxes arrived first before Sovereign purchased any, I certainly remember F358JVS (The last TL11 engined Lynx ever built) being the first I ever rode on and remember being told about the differences when the new G201-7URO arrived (eg having a stepped floor to the rear rather than ramped). Later around 1993 they also aquired F203-8MBT and G384MWX from Harrogate/Keighley & District. More arrived in later years from the KWY and MWU batches.
The long second hand Nationals were aquired around 1992 and the last of them left in 1996 from North Mymms when Volvo B10B's N106-8GVS arrived along with N109GVS and M310KHP at Stevenage. St Albans had 3 B series National's that lasted a few years longer but these were ex-West Yorkshire examples rather than ex-LCNE.
Double Decker wise Stevenage had LR32, 33, 37 and 38 whilst North Mymms had the 4 Bristol VR's. When LR's started to come back down from Keighley Stevenage had LR34 and North Mymms had LR's 31, 32, 33, 37, 38, 39 and 40 as well as 3 Titan's borrowed from BTS until LR66, LR68 and LR69 also arrived back down south.
The ex-Jubilee Metroriders left before the National's in approx 1995, all being replaced by either Mercedes 709D's or 811D's.
I think you're right - Jubilee's takeover was Jan '89, the URO Lynxes arrived in Aug '89.
I'm sure at least one ex Jubliee Lynx spent time with County Bus before returning to Sovereign. County took the ex Jubilee Tiger buses, Sovereign took the Volvo B57.
'96 was later than I thought for the Nationals, but if they were tucked away on certain routes I wouldn't have seen them.