Sadly but long because a lot of problems have been encountered. In the meantime I have received a letter from our user which reported that "3DCoat" works good under Wine from WineHQ repositories. Maybe worth trying that while native version will be built.

File and folder permissions "3DCoat" changes intentionally. We faced with cases when "3DCoat" was unable to read/save particular file and it crashed. But because your request I will add special environment variable which disables such behavior (for example, "COAT_NO_PERMISSIONS").

1. Under macOS "3DCoat" stores your user data (scenes, settings, etc.) inside folder: Finder > Go > Home > 3D-CoatV48. You can backup it or move it to another device. Most probably after updating to latest macOS you will need to install latest "3DCoat" from here (because there were changes to adopt "3DCoat" for newest macOS'es): https://3dcoat.com/forum/index.php?/topic/22364-3dcoat-48-beta-testing-thread/
2. If you have standalone serial number from "3DCoat V4" for macOS you can select "Add another OS" here: http://3dcoat.com/buy/
You will receive additional standalone serial number which will work only in "3DCoat V4" for Windows (the latest build for Windows is in the forum above).

In the nearest "3DCoat" build for Windows the gestures will be supported in "Tablet PC " mode. The video below demonstrates "3DCoat" in "Tablet PC" mode on "Surface Pro": http://pilgway.com/~sergyi/Video/3DCoat-TabletPC-Gestures.mov
The build "4.8.34" with gestures will be here: https://3dcoat.com/forum/index.php?/topic/22364-3dcoat-48-beta-testing-thread/

In the nearest "3DCoat" build for Windows the gestures will be supported in "Tablet PC " mode. The video below demonstrates "3DCoat" in "Tablet PC" mode on "Surface Pro": http://pilgway.com/~sergyi/Video/3DCoat-TabletPC-Gestures.mov
The build "4.8.34" with gestures will be here: https://3dcoat.com/forum/index.php?/topic/22364-3dcoat-48-beta-testing-thread/

These folders are created automatically by "3DCoat" on all platforms and they have such purpose:
1. Folder "3D-CoatV48" contains user data files, like settings, scenes.
2. Folder "AppLinks" is used to exchange with third-party applications/plugins.
3. Folders "3D-CoatV3", "3D-CoatV4" are made for compatibility with some third-party applications/plugins.
To stop "3DCoat" from creating these folder please define environment variable "COAT_FILES_PATH" with full custom path to folder where you want "3DCoat" stores your settings and scenes. Read more here and pay attention to section "Environment variables under Linux" in additional document:
http://pilgway.com/~sergyi/Docs/CustomDocumentsFolder.pdf

We stopped to build "3DCoat" only under old Linux CentOS 6 ("GCC" compiler from old CentOS 6 makes errors during optimizations). But soon we will try to make native "3DCoat" build under newest Ubuntu using its latest "GCC" with all possible optimizations. That should increase speed of "3DCoat" and will make "3DCoat" builds under Linux regular.

We stopped to build "3DCoat" for Linux several months ago because several customers reported that performance of "3DCoat" inside Windows emulator under Linux is better than native build of "3DCoat" for Linux. Regular builds of "3DCoat" are only under Windows and macOS. In time we will try to build "3DCoat" under newest Ubuntu using its latest "GCC" with all possible optimizations. That should increase speed of "3DCoat" and will make "3DCoat" builds under Linux more regular.

According to your screenshots the viewport is sharp. I will explain what happens on example of "Retina" displays under macOS. "3DCoat" supports HiDPI (Retina) displays by rendering more pixels in the viewport. To render user interface elements and text on HiDPI displays "3DCoat" upscales them to maintain the same physical size of user interface on Retina (144 ppi) and non-Retina (72 ppi) displays. I have attached screenshots to explain this. The first screenshot is made on my non-Retina external display (72 ppi). The viewport and user interface are equally sharp because they are rendered without scaling with the same 72 pixels per inch resolution. The second screenshot is made on Retina display of my MacBook Pro (144 ppi). The viewport is sharp (144 pixels/inch). User interface elements are upscaled by two to match those 144 pixels/inch of the viewport. Otherwise the elements would be small. They are blurry because user elements and font textures in "3DCoat" have resolution 72 pixels/inch. Note that "3DCoat" doesn't have any special code to support HiDPI under Windows. "3DCoat" doesn't upscales its user interface under Windows in any case. That "upscaling" becomes from Windows system setting "Change the size of text, apps, and other items" which effects in the same way as I have explained above.