So IF you were to go see him and say "I heard you've seen Githyanki, I need to know where..." he wouldn't know what you are talking about with out a dialog option you give a detailed description of Lae'zel... so you see it becomes very convoluted very quickly... do able, but a tad convoluted...
That is certainly one way to put it ...
Or you can simply go to him and say:
"I head you have seen some odd beings ... i believe your description was exactly "yellow as a toad and twice as ugly" ... I would like to ask you where have you seen such creatures?"
Or you can even improve your chances for positive reaction by saying:
"I just helped your friends to kill one of them ... Nemisa and Deneis was their names i believe?"
Or you can add [deception] roll to the conversation by saying:
"They killed one of my companions ... i seek revenge." (that should be quite easy roll btw

)
The point is ...
You have found sentences that doesnt work ... good, now we know wich way we dont want to go ...
But that certainly does not mean that working sentence dont exists.

So... Without Wyll there for the interaction the only real dialog that could be added would be something along the lines of "The Blade of Frontiers says hello" attack.
Again, that is not necesarily true ...
I mean sure *you* dont know about Myzora, that is true ... but Spike do ... and he seems (at least to me) like the type who A) dont watch his tongue and B) likes to brag ...
So i could easily imagine situation that you mention to him that you know Wyll ... and spike tells you something like:
"So you know each other ey? Well, next time you see my pussy cat tell him i had my fun with his pretty lady. Shame she is allready gone, her scream was delicious."
Or something simmilar.

Nice and easy.

The same holds true for other interactions. Parts of Shadowhearts story are just going to be lost if she's not in the party with you because without her there to interact with environment certain cut scenes just don't happen.
This one is true ...
But since you are not suppose (ignoring the armor, and her clear despite of Seluna) to know that she is Shar worshipper ... it makes sence that you are kept in the dark.
In both Lae'zel and Wyll case you KNOW what they are after (even tho in Wylls case you only think you know and in the finale it shows up that you were after false goal this whole time ... but still, you know enough to interact).
If you want another example, its Astarion and that Monster Hunter in the swamp.

I dont think the game would lost anything from its value ...
With Lae'zel you would loose whole Githyanki interaction if you dont take her with you ...
Even tho you could question Zorru yourself.
With Wyll you would loose whole Spike interaction if you dont take him with you ...
Even tho you can still find out about Myzora.
Also ... it seems to me that it works better for replayability if you can simply pick your party and play ... instead of switching back and forth for every conversation your companion have to present on ... :-/