I found a website saying you can only get Mew in FireRed by trading Mew from Emerald.
Yes you can rematch greevil at anytime and he will still have his shadow Pokemon if you defeated them.
The game is technically over but you can still collect Pokemon and go to more places.
It is still in Cerulean City. However, you can only battle her after solving the problem at the Power Plant. After that, you will find her on the bridge in front of Bill's Grandfather's house. Talk to her, and she will return to the gym and battle you.
If it is a ledgendary Pokemon (meaning there is only one of it in the game), them no
You will only have one chance to catch Lugia so if you make it faint, you will not be able to catch it again. However, if your whole party faints, you will go back to the Pokemon center and you can battle it again.If you failed to catch Lugia, you can still trade for it.
you can still go to cerulean cave to catch it like in firered and leafgreen
Mew is not in in Pokemon SoulSilver. There was a Mew event however that has ended on February the 11th. You can still catch a Mewtwo in Cerulean Cave at level 70.
After beating the Pokemon League, you still can't go to the Cerulean Cave. To be able to enter you have to get the National Dex by catching 60 different types of Pokemon and showing your PokeDex to Professor Oak, also you have to do the Ruby Sapphire side quest in the Sevii Islands and beat the Pokemon League once more when their Pokemon are stronger and different. By then you will be able to enter the Cerulean Cave that contains Pokemon levels 50-60 and Mewtwo Level 70 at the Bottom floor of the Dungeon.
All you need to do is get all eight badges and defeat the Pokemon League. But, you can decide your goal to beat the game. You still have to beat the Pokemon League, but afterwards you can decide on your goal. Your goal could be to catch all Legendaries of that game. You goal could be to catch a number of Pokemon. But, the true way to beat the game is to beat the Pokemon League.
if your party is not full
there's still plenty to do! you can catch latios/the regis/(raquaza if you haven't already), challenge the battle frontier, and if you haven't already-contests as a side note, contests are not suited for fighting Pokemon
Beat the elite four and if he is still there obtain at least 60 pokemon data
nothing really unless you still have more Pokemon to catch POKEMON GOTTA CATCH 'EM ALL!(or do you?)
yes
You can't catch a hymen if you still play Pokemon.
Yes you can rematch greevil at anytime and he will still have his shadow Pokemon if you defeated them.
Unfortunately, you have to catch, trade, or somehow own every Pokemon in the pokedex. If you had it at one time and traded it, it still is in your pokedex.