Is it possible for different groups landed on the same time on the island to run different timelines?
The answer is yes! This may explain why Rousseau lived 16(?) years on the island without confronting the Others, without discovering the Darhma stations (which discovered by the survivors in a couple of months). This may also explain how Inman and Desmond remained untouched inside the Swan station long after the Others gained control over the island.
During the 815 crash, did all the survivors "landed" in the same timeline? May be not! If Christian or Christan's body landed earlier than his death, it does make sense to be visible. (Actually, In the same way, Mr.Eko, Charlie, Claire and anybody who is "dying" on the island can be seen because their "formal" date of death is yet to come in the future. Especially, if the dead body is unburried. The bodies of Anna Lucia and Libby have been buried, but only Hugo can see and talk with them.
The inconsistent timelines on the island is the big problem which must be fixed. The aim of the Oceanic 6 is to avoid the inaccurate deaths but also the improper survival, in order to die properly, all together, in the crash of 815, somewhere west of Indonesia.