SBT plugins not using custom resolvers

Yes this is a correct way to define your resolvers for plugins
This is actually quite tricky : you are trying to share settings between the plugins build and the build. These are "separate" projects from sbt's perspective. There are solutions using a project ref to gain access to the plugins build settings from the build but they are quite tricky. If this is a corporate environment where you will never have access to the default repositories anyways, it might be easier to use a custom sbt.boot.properties. You can start from the default file for 0.13.x Changing the sbt.boot.properties should deliver you from the Server access errors.
There is nothing in the logs you provide which indicates that the plugin failed to be resolved from your proxy, only that it timedout trying to reach the official repositories. With the default sbt.boot.properties, sbt will try to resolve artefacts in the order the resolvers are defined :
```
local
typesafe-ivy-releases: https://repo.typesafe.com/typesafe/ivy-releases/, [organization]/[module]/[revision]/[type]s/[artifact](-[classifier]).[ext]
maven-central
sonatype-snapshots: https://oss.sonatype.org/content/repositories/snapshots
```
Then any resolver found in any .sbt files in the <project_home>/project/directory. As far as I am aware, sbt files are evaluated in the alphabetical order and resolvers are added in the order they appear in each file.

Related