The BAe146 is an interesting aircraft in several ways, but for ditching it is similar to many others. Some very early build 146 aircraft were not equipped with the ‘ditching mod’; actually all this meant was that the initial rear fuselage design strength did not meet the certification requirements, this was quickly rectified, but the structure changes not necessarily retrofitted. Aircraft without the mod will probably be restricted to operations within 85 nm of the shore i.a.w. other regulations.
I am not aware of the particular restriction quoted, but there may be different restrictions on which door to use between the different lengths of aircraft 100/200/300. Rear door restrictions are not uncommon for high wing aircraft; the wing does not play any part in the initial floating capability. Re Capt S post, the F50 uses ‘barge boards’ to dam fast water entry into the cabin via the rear doors.
Evacuation requirements are based on only a proportion of the doors being available; thus there is a case for inoperative doors in any MMEL. For ditching there may be a specific case in the 146 MMEL that considered the extent of the rear door restriction i.e. the floating water level is only 1 in above the door sill (does not meet the certification requirement), but for practical purposes and the extremely remote probability of ditching when operating with a rear door on the MEL it is perfectly acceptable and safe to operate in this manner for short period of time.