I'd recommend a couple of things...
One, don't worry about "fancy" stuff like barrel roll merges etc. They'll mainly bleed your E, and won't do much to actually confuse someone who has any idea at all of what they should be looking for on a merge.
On your "normal" merge, I don't recommend you veer so far to the side, and I don't recommend you ease off throttle. You don't need that much lateral separation, and as often as not, it will hurt you. Easing off throttle is a bad idea at this point as well, because your just wastefully burning energy. I'm not saying you need to be haulin' azz, with so much speed you can't be effective either. Regulate your speed by controlling your dive instead.
Timing... You sound "late". Ideally, you'll recognize that dot on the horizon as unlikely to be friendly before you ever see the icon, and will have a good idea of whether you intend to attack him or not. That said, by the time I see the icon, I "know" what my plan is, and will begin setting up my merge immediately. By that, I mean begin my dive by 5.5k out. That gives me some options when it comes to dive degree, and therefore speed. I want to be "level" by 2k out, and under my opponent, just slightly to one side (so he'd need to kick rudder for a shot on me). By 1.5k out, I'm already coming back up at his tail. My "intention" is to come up as close behind him as possible. Ideally, he'll be diving in while I'm coming up at him. Most guys will pull an immelmann as they pass at the merge, regardless of whether it's a good option for them.
In the case above, being below your opponent with speed is an advantage. Pulling up aiming at a point right behind your opponent (essentially beginning your immelmann) as he dives at you prior to beginning his own immelmann is an advantage (you've already begun yours, he hasn't). He's also "following" your lead, which is another advantage (you're dictating the fight). Done perfectly, with a willing opponent, you'll come over the top of your immelmann, pointing slightly nose down, and have a shot right into his cockpit as he's still going up in his immelmann. Done not quite as well, you'll be able to dive slightly, and pull up onto his six as he comes over the top of his immelmann.
Back to your initial merge description... AS you see he's doing what you're doing you roll level and pull up... That sounds suicidal. All he needs to do is pull up onto your six.
In the case where the other guy does what you're trying to do, there are a few options I use.
One, beat him at the game, by starting my dive further out, and getting under him anyway.
Two, let him "win" the low position (but don't make it obvious, act like you want it too). As he passes you, pull up just slightly (10 degrees or so) so if he's watching, he'll crank into his immelmann. Then you just continue on your course, in a slight climb (no immelmann for you!). As he comes over the top, bring you nose up gradually, and draw him into a rope. He follows you up, stalls below you and you come over the top and shoot him. If he won't follow you up, you now have the alt advantage and can attack from above (this is also a useful trick to get a faster plane to commit to burning a bunch of his E).
Three, trick him into taking a bad merge (riskiest choice). Do this by simply letting him "win" the low position again, and then doing a flat around turn to the left. While he's doing his immelmann (and likely near blackout with no vis on you) you're flying a flat 1/2 circle at your merge height (you MUST stay below him for this. You also want to came around as fast as you can, but retain as much speed as possible- merge number two is coming up!). As he comes to the top of his immelmann, and reacquires you, he'll point his nose down at you. Meanwhile, you nose up at him... Bingo! Merge number two, this time with you below him and nose up, him above you nose down. A variation of this is to pull onto his six right away as he dives past you. As he comes over the top, and dives back at you, he'll almost always pass by on your left. JUST AS he does this, you roll RIGHT, and pull into a dive right behind him. This "merge" often works well because as your opponent comes over the top of his merge he's slow, and then as he dives in on you he doesn't have time to speed up much, but tries to pull another immelmann. It's dangerous though, because that big flat 1/2 circle bleeds a ton of your E, and if he sees you do it he can just zoom way up out of reach, and then attack you from above. One possible advantage this merge has is that it makes you look like an inexperienced pilot, who doesn't know that a flat turn is a bad merge choice. This seems to cause lots of pilots to rush into that second "merge", when they really shouldn't.