If some 3D models are missed you can search for them on https://www.3dcontentcentral.com/

Please update to the latest build (5.1.0.3 at the moment) and let me know if you still have the issue.

There is the only net (Net 0) in your design. You can see the Net 0 only in the design manager. Also you can hover mouse pointer over any existing trace and see that all traces are highlighted, i.e. they all belong to the same net.
I think, you merged different nets into the only net by accident. As ...

It's common to have ground and power planes on internal layers of 4-layer PCB. Outer layers are signal. This is not "must have" but very popular solution.
Based on the stackup you can calculate trace width and primary gap for 90 Ohm impedance.
There is no simulation of the impedance matching in ...

Bill of materials can have as many columns as you need. You can create an additional field in component properties. Then you will be able to add new column in the BOM and show the value of the additional field in this column.

There is no way to add a specific rotation offset to a pattern when attaching it to a component.
Although you can add an additional field in component properties and write a specific rotation offset into the field I don't know how you can use it further.

Thank you for the suggestion, we will consider it.

We depends on Wine at this point. It's unclear whether x86 programs will run without Rosetta2 or not.
We wanted to have native version of DipTrace for Mac OS but it would require huge amount of work thus delaying core features development of DipTrace. We will consider our options when closer to EOL ...

Blinking an entire net with one second intervals using a plugin is not good idea as it requires periodic reading and overwriting a file thus it loads SSD or HDD significantly.
1) Using the design manager a net is highlighted but not selected.
2) What exactly do you want to do with a selected segment ...

You can't merge (short circuit) two different nets into one net if "push traces" option is on. In that case, the solution is turning off the "push traces" option by pressing "I" hotkey while manual routing.