I'll try to answer you Q's line-by-line :-)
So far what I got is, 'shika' can only be used with negative verbs. even though it means 'only', the real meaning would be 'anything but... didn't....'
Is this right?
>> shika is only for negative sentences.
The examples say
a) 'John shika konakatta' - 'Only John came' ?
>>That's correct. But it is implying that many others were expected to come but it turns out that only
>>John came. So it is unexpected or even a little disappointment is implied there.
If I wanted to say, 'only john DIDNT come', should I use 'dake' like:
b) 'John dake konakatta' ?
>> This means only John didn't come. But this implies also that everyone else who were expected
>> to come came, except for John.
Then, why can't I use 'kedo' as 'only' too?, as in
c) 'John dake kita'
>> This means only John came. But this implies that there were other who were expected (or invited)
>> but only John turned up, and nobody else did.
What if i said:
d) 'John shika kita' - would it mean anything at all?
>> This sentence is wrong; either John shika konakatta or John dake kita, as explained above, makes
>> sense and correct.
do 'dake' and 'shika' replace particles? Let's pretend i want to write
'you're the only one i want to meet'
e) ' kimi shika aitakunai no '
be right? ( the 'no' ending softens it in an emotional way, right?)
>> I want to meet only you (and nobody else). Yes, "no" is an affirmative particle which does not add
>> any meaning but makes the sentence less direct or softer, slightly.
Now what about igai?
>>Igai is equivalent of "except", or "other than". John igai kita means everyone except John came.
I hope this helps. Keep asking interesting grammar questions!