What seat height is feasible for me (Supermoto)? Apart from lowering the bike, adding padding, and wearing extra shoes, are there any tricks I can use to cheat a few centimeters?

So first of all, xo0ox has mentioned it already, a SM is pure eyewipe without practical use. From driving, the Enduro is always a little better, no matter where you look at it.

Second error: seat height. This is not the height at which you sit, but the distance from the bench to the roadway with unloaded moped. Now seat padded to crumb the characteristic, otherwise they would not be upholstered. And a suspension should compensate for unevenness on the road. There are unevennesses not only up (lifting) but also down (holes). For this reason, in the basic state, a suspension must not be located at the top but somewhere in the middle. so that it can react in both directions.
This is why the so-called sagging is set in such a way that the moped when the driver/driver sits on it by about 1/3. This makes the WR even lower by 6cm + upholstery.

3rd point: Husqvarna. Unfortunately deceased. The owner of KTM, Pierer Holding bought Husqvarna from BMW 3 years ago and just kicked out all the people and made the shop. The manufacturer Husqvarna no longer exists. That was a very brutal resolving of competition.
In order to render the action more publicly positive and to hide that KTM has sought and almost achieved an off-road monopoly, they have transferred the brand name to their own Husaberg product line. Husaberg finally eliminated them.

So if you buy one of the old Husqvarnas from Varese now, you have a bike for which there is no support or spare parts. Not a good idea.

4. Leg length. If you come down from 85cm with the ball, you'll come down from a meter when you set the pelvis obliquely. Higher mopeds are not known to me.
If you're moving in with a slanted pelvis, you're not coming down on the other side, okay. In the solitary history of mankind, there has never been a mop at the same time, so what?

As for the WR. On the one hand, they really drive around dozens of times at every corner. And it is also relatively heavy.

Look at the beta models. They are similar (one is also somewhat deeper) as the WR125R. But they are slimmer and lighter and easier to launch.
And they have longer spring paths, so come down again 1-2cm when correctly adjusted sagging.
By the way, the engines are the same for Beta and Yamaha, from Yamaha-Minarelli in Italy.