Merlin's Death is generally not due to the cards, but to play style. However, in general Evil needs to keep track of things, so the more 'confused' evil can be the better. The best configuration for this (which I would try in your group, first) is Merlin, Percival, and Oberon. This gives good maximum information and evil the least. If this configuration affects the Merlin kill, then great, but I doubt it will much.
The bigger problem seems to be that "Good generally wins". This shouldn't be the case. Merlin is there to balance the information on both sides, since Evil knows Evil. In general, games should end 50/50. Merlin giving out too much information should be the only reason these scales tip away from 50-50.
As for question #2, how do you steer things? The short answer is you don't! The biggest reason for Merlin kills is not based on how Merlin behaved, but how all of the other good players behaved. My guess is all the rest of your loyal knights are acting timid and confused, while Merlin is acting confident, bored, or aggressively. You need to be aggressive, talkative, and opinionated in order to get the information out there to the other good players, ergo, the other good players also need to be aggressive, talkative, confident, opinionated, etc. If they're not doing that, there's very little that can be done by Merlin. I call this "giving Merlin cover".
Note, this isn't just Percival's job. It's everyone's. Percival is there mostly for late game use when you need a perfect team on quest 5. Think "I'm Percival, this is the team you need". He knows this because he's been carefully watching Merlin all game and Merlin has communicated to him who the bad players are (in some way).
As a normal loyal servant of Arthur, your job is pretend you have all the information and any 'mistakes' you make are a ruse to throw people off the fact that you're Merlin. This usually means a wild guess. However, these guesses often pay off. With 2-3 loyal knights guessing maybe one of them is accurate and this gives Merlin the cover he needs (as Merlin too will look like a random guesser).
Finally, don't discount the power of voting. The game relies on people voting in their best interests, as such, voting is a VERY reliable indicator of someone's knowledge and preferences. If you're being fingered as Merlin too often and this is because you're speaking, try switching up the strategy. Instead of speaking, try being totally silent, but just consistently voting for all good teams and down voting all bad teams. If the other players are paying attention to votes you can give full information about team evil without saying a word. Of course, at some point they might catch on to this, so you might need to change-up the strategy.
When I'm Merlin, I play exactly the same way as when I'm not. For me, this is trying to do a logical tally and deduction of votes and to point out table behavior and it's consequences. Usually, even without being Merlin, I can fully figure out Team Evil by the end of the game with this method, so when I do it as Merlin I need to carefully act the same. Still, in the absence of any cover, I will still lose.
Things you generally shouldn't do as Merlin (though deception always allow for exceptions):
- Vote for losing teams. This 'fools evil' far less than it screws good.
- Approve team with 2 Evils early. While this is a sound strategy, it can confuse good players who are trying to find you, and can convince them an evil player is good.
- Claim to be Merlin at the end of the game. This ploy generally doesn't work.
Things that are perfectly okay to do as Merlin:
Claim to be Merlin near the beginning of the game. I've used this strategy effectively several times, even pointing out the exact bad guys. It's often so bold that it fools the opponents, but, of course, it can only be used sparingly, and it too needs cover (e.g. by players randomly doing this in some games)
Speak your concerns. It's okay to think that a bad guy is bad and say so. Often a Merlin 'tell' is being afraid to 'give away' anything, as that fear will come across and garner suspicion.
'wink' to Percival. While perhaps not a literal wink, Percival is going to be looking right at you, so dammit look at him and acknowledge each other! This is especially important in games with Morganna.
Overall this is a game about information and mis-information. At the start Evil has an information advantage, so that needs to be made up with by good player speak. Likewise the thing evil doesn't know (Merlin) needs to be covered up by a lot of misinformation from Good. The more stuff you put out there, the more Evil needs to consider and the more cover Merlin will have.