Is it possible to force a Mac *.webloc file to open in a new Chrome window instead of a new tab in an existing window?
Here is an option to consider...
Use an Automator Service/Quick Action1, via Finder, to open the URL contained in the .webloc file in a new window of Google Chrome.
1 In macOS Mojave, and later, an Automator Service is called a Quick Action. The use of terms separated with a / is to denote the differences between Automator prior to macOS Mojave and from it onwards.
The Automator Service/Quick Action can be triggered from the Services context menu via right-click (option-click) on the selected .webloc file(s) in Finder, or after selecting the .webloc file(s) in Finder press the keyboard shortcut assigned to the Automator Service/Quick Action.

In Automator create a new Service/Quick Action, setting: Service/Workflow receives/receives current [files or folders] in [Finder]

Add a Run AppleScript action, replacing the default code with the following example AppleScript code:


on run {input, parameters}
    repeat with i from 1 to the length of input
        set thisItem to the POSIX path of item i of input
        if thisItem ends with &quot;.webloc&quot; then
            set dssCommand to ¬
                &quot;/usr/libexec/PlistBuddy -c \&quot;Print :URL\&quot; &quot; &amp; ¬
                quoted form of thisItem
            set thisURL to do shell script dssCommand
            if thisURL starts with &quot;http&quot; then
                tell application &quot;Google Chrome&quot;
                    set the URL of the active tab of ¬
                        (make new window) to thisURL
                end tell
            end if
        end if
    end repeat
end run


Save the Automator Service/Quick Action, e.g.: Open webloc in Google Chrome

Add a keyboard shortcut in System Preferences &gt; Keyboard &gt; Shortcuts &gt; Services, e.g.: ⌘G



Notes:
Depending on how the .webloc file was created, it can be either a binary PLIST file or an XML plist text file. The examples below shows https://www.google.com/ as the URL:

&lt;?xml version=&quot;1.0&quot; encoding=&quot;UTF-8&quot;?&gt;
&lt;!DOCTYPE plist PUBLIC &quot;-//Apple//DTD PLIST 1.0//EN&quot; &quot;http://www.apple.com/DTDs/PropertyList-1.0.dtd&quot;&gt;
&lt;plist version=&quot;1.0&quot;&gt;
&lt;dict&gt;
    &lt;key&gt;URL&lt;/key&gt;
    &lt;string&gt;https://www.google.com/&lt;/string&gt;
&lt;/dict&gt;
&lt;/plist&gt;

Regardless, the example AppleScript code uses a do shell script command to assign the value of the URL to a variable by using PlistBuddy to print the value of URL in the .webloc file. It then opens the URL in a new window in Google Chrome.
The PlistBuddy full output for the example .webloc file, in both forms, above is:
Dict {
    URL = https://www.google.com/
}

However, when configuring it to print the URL directly, its output for the example .webloc file, in both forms, above is just:
https://www.google.com/

As coded, it can handle multiple selected .webloc files opening each in its own window, regardless of how the preferences are set in Google Chrome. It validates that the file(s) passed to the service have a  .webloc file extension, and the value of the thisURL variable starts with http, otherwise the service fails silently.

This Automator Service/Quick Action was tested and worked for me, under macOS High Sierra and macOS Catalina, using both the Services context menu and ⌘G assigned keyboard shortcut of the service on the selected .webloc file(s) in Finder.