Horrible sadness

I have read this and thought about this situation.  Amazing that you didn't find any evidence of the kitten being hurt in your yard.  Although it doesn't surprise me that two dogs may do this, when they are together it is like they share a brain that doesn't function altogether that high in intelligence!  Instinct would be the cause.  Dogs are dogs.  However, you have found no evidence. 
 
Sorry about losing your kitten.  Forgive the dogs, they are just dog brains... They chase little animals and if they don't attach "pet status" to it, who knows.  Even kittens grow up to chase littler animals and kill them, after torchering them. 
 
And perhaps they didn't do it.  In any case, losing the kitten was unforseen, and not on purpose.  It happens.  I hope you will find it. This must be hard for you.  []

Prey drive is engaged by *motion*, and the dogs, once something small and furry runs by, have little control.  They just go.  They make no conscious decision, and so it is not fair to hate them for something they were born hardwired to do.  The fault is not with them.
As to your dog having the seizure, it could easily have been triggered by a high state of arousal, which the chase certainly is.  Your kitten was a feral kitten, and had he not been rescued to live a short while in comfort, could certainly have met the same end at the hands of a fox or a coyote, who would be acting, just as dogs do, on instinct alone.
Dogs cannot tell the difference between a rabbit and a cat if it's running by them lickety split.  Some dogs will even down a cat, then realize it's their own, and let it go.  But, your dogs had no lengthy exposure to this little guy telling them it was a "packmate".  To them, he was just a prey animal. 
My advice would be to stop fostering any cats, since you know there are dogs in your home that are predatory, and just stick to dogs of the same relative size (no Chi's or Yorkies...etc.).  And, stick around!

This is a very sad situation... my cousin just went through something similar recently.  To make a long story short, one of her dogs (Chloe) caused injury to their older dog (Duke) and they had to put Duke to sleep.  It was horrible and Duke should not have had to endure that kind of pain at the end. 
 
All I can say is... I wouldn't give up hope on Spencer and I definitely wouldn't stop loving my dog, especially if I didn't see it happen for myself.  My cousin still loves Chloe and wants what is best for her... I know the shock of it is causing all of these thoughts to run through your head but don't give up on your dogs.  They need and love you.

Just FYI, I talked to my neighbor on Saturday & showed him a picture of the kitten. He assured me it wasnt the same thing he saw Marley with.
 
So I guess Spencer has gone from missing & presummed dead to just missing.
 
I still have a feeling the dogs could have killed him, there wasnt a body anywhere though. I know if they killed a cat it was Spencer. The neighbor is sort of "loopy" so maybe he saw Marley with a toy or something.
 
Anyway, I am on the lookout for him. I put flyers up a couple of places and am trying to figure out where else would be useful.
 
Good luck trying to find a cat in a city though. I am thinking about setting up another trap to where if he is in the area I might get him. But I might also catch some feral cats instead. Some of the girls down the road said they might see him but asked if he looked like an adult cat which I am not sure what that means? I showed them the picture and told them it was a kitten. But they said they saw a kitten who looked like him running in the field down the road from where I am. I looked for 20 minutes or so and didnt see him. I am not sure if he would come to me anyway, he might be too scared. I wondered by their comment if they didnt mistake Cuddles (one of my other cats who thankfully I think I have finally gotten to where he cant get outside) for Spencer.
 
I have been leaving cat food out & it was gone this morning. One of the feral cats could have gotten brave though and eaten it. (The food was very close to the house)
 
SO I am just keeping up hope. I thank you all for your advice & words of encouragement. I am obviously still going to be involved in rescue considering I am picking my new foster dog up this week. I might wait a while to take in any foster cats and even then I might just take in full grown ones to be safe. (Hopefully Spencer will come back so I can keep him and I will be full on my "cat limit".)
 
Spencer:
 
 
 

So is the situation with the dogs better??

Another thought that occurs to me is that your neighbor probably doesn't like cats -- "bagged" isn't a word *I* would use to tell a neighbor something like that. I'm not sure I'd believe that story as it came out.

 
Ditto. We accord criminals caught on tape the notion of being innocent until proven guilty in a court of law. Can we extend the shadow of a doubt to the dogs? If the dogs had killed the cat, it would either be laying where it could found or it would be freshly buried. If there are no new big holes in the yard, it is likely the cat ran off.
 
Shadow bit me once and it was totally my fault in every respect. Not once did I get mad at him or punish him or regret having him. I committed a doggy faux pas. And if he were to take an unliking to our kitty, I would equally not get mad at him and the reason I was a bit cautious about rescuing the kitty is because I didn't want DW to be mad at him if he were prey-driven toward the kitty. A dog is a dog.
 
But I like kittens, too. I hope that either he comes back or that he is doing well, whatever may be the case.