It will use the Index Sample (RAM) first, then it will use "full" Index (disk) and finally it will read data from SSTable (disk). There's no such thing like "collision" in this case.
2. when cassandra lookups a key in sstable (assuming bloom-filter and other
"stuff" failed, also assuming the key is located in this single sstable),
cassandra DO NOT USE sequential I/O. "She" probably will read the
hash-table slot or similar structure, then cassandra will do another disk
seek in order to get the value (and probably the key). Also probably there
will need another seek, if there is key collision there will need
Yes, this is how I understand it too.
3. once the data (e.g. the row) is located, a sequential read for entire
row will occur. (Once again I assume there is single well compacted
sstable). Also if disk is not fragmented, the data will be placed on disk
sectors one after the other.