Poor Toaster! First he has a new "baby" to adjust to in his personal space, and the attention that kittens inevitably get will have reduced the amount of quality time he gets with you, so he's feeling doubly insecure about his position in the house. His jumpting ont he work tops is probably his way of (a) getting away from Kevin and (b) him saying "I'm here, please notice me. Don't you love me anymore?" Annoying as it is, him being on worktops is not the end of the world, so I'd stop stressing about it. Some battles just aren't worth fighting, particularly if then getting told off more, when his world is already turning upside down, is stressing him.
Kevin is now moving into early adolescence, so imagine having a hormonal attention seeking and lively 13 year old boy in the house (the horror!). Toaster doesn't sound like he's aggressively attacking him, he's just doing what cats do, ie establishing a hierarchy. It's very rare that cats act like best buddies in a way that we would understand, but so long as Kevin isn't unduly stressed or harmed by this (and his submissive pose sounds like he's just accepting the lessons from his elders and betters), I wouldn't worry about it and leave them to it. Yes, it's a bit sad to watch, but once they've reached an understanding and Kevin learns to treat Toaster with a bit of respect, they'll soon learn to give each other space and cohabit relatively peacefully. You could step in when it looks like it's getting out of hand, but don't punish either of them, just separate them and give them a few minutes apart to cool down. I often found just a
gentle tap on the back of both parties was enough to make them look away from each other (ie at you) for long enough to give one of them the opportunity to exit the situation with some dignity, and they could retreat to their corners to calm down.
Meanwhile, do give Toaster some love and reassurance, and instead of getting cross with him when he's on the worktop, gently put him on the floor, saying no in a firm (but not necessarily loud) voice, then
immediately give him some treats on the floor to reinforce that this is where good things happen, and
give him some love!
The pooping outside the box is a territorial thing. Some cats do it in the garden when newbies are on the block to tell them, more forcefully than by simply spraying, that this is Their Turf! Once your boys have adjusted to being together, this should stop, but meanwhile keep the litter trays clean and perhaps put some newspaper on the floor, or a bit of old floor vinyl, to make it easier to clean up. I know it's gross, but my old boy Mitz did this nearly every day towards the end of his life, (and I'm not the sort who would normally take cat toilet problems in my stride), and I just learnt that it was associated with stress and just cleaned it up. It wasn't the end of the world. In his case, the stress was associated with his blindness, so there wasn't a lot I could do about it, but in your case hopefully this should improve once Toaster is feeling a bit happier.
You might find this link helpful in understanding the dynamic between them, but Vicky Halls has written some good books on cat behaviour, and "Cat versus Cat" (below - "other good booksellers are available!"

) is also very good, and easy to read.
http://www.catchat.org/hierarchies.html
http://www.amazon.co.uk/Cat-vs-Keeping- ... 0142004758