Reducing traffic noise through construction design and sound barriers

The closed noise protection wall in the form you drew will definitely be effective.
On laermorama you can calculate some values, roughly in that range the reduction will actually be - if the wall is really continuous and made of a suitable material. Depending on wind and weather, the wall can sometimes be a few dB more or less effective, but it is definitely useful. You just have to accept the wall itself and possibly the shadow it causes.

Nothing at all. You won’t manage it just because of the reflections from the surrounding buildings. Take a look at sound propagation. Noise barriers as an active noise control measure are only suitable close to the emission source at reasonable heights for reducing the immission levels. The further the barrier moves away from the source towards the receiver, the higher it has to be to have any effect. In your case, that means building-high. If it weren’t for the reflections.

Try simulating on various sun position websites whether the shadow cast by the wall at the planned height will be relevant at all. If your plan is oriented north, it could actually be quite good as planned. How high is the wall planned compared to the terrace?

I could also imagine that you will first only build the left half of the wall on the plan. Then you can see if any significant noise comes from the right side behind the house. That might already be sufficiently shielded. If you are very noise-sensitive, then build it completely right away.

Nothing at all. You won’t manage it alone because of the reflections from the surrounding buildings. Familiarize yourself with sound propagation. Sound barriers as an active noise protection measure are only suitable near the emission source at reasonable heights for reducing the immission. The further the wall moves away from the source towards the receiver, the higher it must be to still have any effect. In your case, that means building-height. If the reflections were not there. Align buildings more in a north-south orientation, like 87 and 87.Terrace facing south. As I said, 15 dB is utopian. As I said, given the distance to the emission and reasonable wall heights… probably not. Maybe psychoacoustically, but the measurably verifiable effect will be limited.

The greatest effect you achieve if you advocate for a 30 km/h speed limit in the town and a bypass. You will not significantly reduce the noise for yourself through construction measures. You are right that noise has a negative impact on health. There are options - some parties and candidates address this issue both locally and regionally. Everyone has to weigh this up for themselves if they are not a "single issue" voter.