I’ll give you answers that reflect my point of view. Not necessarily correct…
1/ yes SI2 and SI5 can be different. No logic behind, except is you plan to prevent HO between two cells, but want to keep reselection available.
2/ no logic behind having 2 different SI messages. But keep in mind that on SI is sent on BCCH, the other is sent on SACCH.
3/ long list of neighbors lead to problems :
– you can’t use co channel or adjacent channel BCCH between neighbor cells. It makes your FP more difficult.
– the MS will spend a lot of time scanning the whole amount of neighbors. It will select the 6 strongest, ok, but it has to scan all of the neighbors to find the 6 strongest. Therefore it might miss measurements, and will not be able to update the RxLev(n1…n6) every 480ms.