1) Correct - although certain languages require specific main firmware, too - you can get away with mixing things up quite a bit, but it's safest to keep everything matched.
2) Yes - if you are changing the model, then you should reflash everything, and any patches will need to be reapplied.
3) This is normal - the SIM lock state is being read from the security zone, and that is still locked - but the patch means that it's ignored.
4) The "K610" is the basic model number - the "i" suffix indicates that it's the "international" variant - there is also a "K601im" that has i-Mode support. Some other phones have "c" (China) and/or "a" (America) variants. If something (like a firmware) is marked simply as "K610" it means that it's applicable to all variants.