I understand that an observer outside the event horizon (EH) of a black hole (BH) will not see anything disappearing from outside the EH - only the effects of the time dilution near the EH.
He'll see gamma ray bursts. People tend to forget about gamma ray bursts, but they're important. It was the detection of gamma-ray bursts, by satellites checking for Russian nuclear explosions, that reawakened interest in general relativity in the 1960s.
Assume that a large number of objects are on its way towards the EH and the last objects can observe all the others. It is right to assume that the all – from the last observer’s viewpoint – cross the EH at the same time?
No. Not at all. See Einstein's 1939 paper on a stationary system with spherical symmetry consisting of many gravitating masses. He said this: “g44 = (1 – μ/2r / 1 + μ/2r)² vanishes for r = μ/2. This means that a clock kept at this place would go at the rate zero. Further it is easy to show that both light rays and material particles take an infinitely long time (measured in “coordinate time”) in order to reach the point r = μ/2 when originating from a point r > μ/2”. Unfortunately Einstein missed a trick here, in that falling bodies don't slow down. They fall faster and faster. As to why, take a look at this: "As a simple geometric consideration shows, the curvature of light rays occurs only in spaces where the speed of light is spatially variable”. That's Einstein again. So your observers are falling faster and faster towards the black hole because the speed of light is getting lower and lower. If this continued, eventually they'd be falling faster than the local speed of light. That can't happen because of the wave nature of matter. So something else has to happen. A gamma ray burst. Your observer sees all the other infalling observers erupting into gamma rays bursts one after the other. BOOM! BOOM! BOOM! See Friedwardt Winterberg's 2001 paper gamma ray bursters and Lorentzian relativity. This was the original "firewall" paper. It's reference 87 in an apologia for firewalls.
A natural follow up issue is the pattern the observer sees thereafter, i.e. inside the EH.
He doesn't see anything. He's long dead. I know people say he doesn't observe anything unusual as he crosses the event horizon, but that's based on a version of general relativity that contradicts Einstein. See this image from page 848 of Gravitation by Misner Thorne and Wheeler:
Look at drawing (a) on the left. The vertical dashed line denotes the event horizon. The curve on the right of the vertical dashed line denotes the path of the infalling body outside the event horizon. It gets closer and closer to the event horizon as the time t increases. Note though that the chart is chopped off at the top, and the infalling body somehow crosses the event horizon at time t = infinity. It somehow jumps over the end of time. Then the curve on the left of the vertical dashed line denotes the path of the infalling body inside the event horizon. The drawing (b) on the right shows Kruskal-Szekeres coordinates where the t = infinity has been swept under the carpet by the use of "tortoise" coordinates. That's where seconds last for a longer and longer time, eventually lasting for an infinite time. Amazingly, people take this sort of thing seriously.