When do you experience a crash? When using a specific tool? When working with specific files? Are you prompted to send a crash report and are you sending those reports? If so, are you including your email address?
By default, the Mixer Brush is where it's always been: nested with the Brush tool. However, it is possible to customize your UI:
- To customize just the Tool bar, go to Edit > Toolbar
- If tool has been disabled, it will be listed on the right under the Extra Tools column. Simply drag it back to the left and put it with whichever tool stack you want (default is with the Brush tools)
- If it is not on the right, it will be listed somewhere on the left. If it is not where it belongs, simply drag it to where you want it
- There is a Restore Defaults button on the right if you just want things back to factory settings
- To customize the all panels, including Tools, go to Window > Workspace
- If you have something other than the default Essentials selected, some of the other options disable the Mixer Brush. You can change to a different workspace.
- If you are in a workspace that normally uses the Mixer Brush, you can restore the defaults for the workspace with "Reset [Name]"
- See the following for more: https://helpx.adobe.com/photoshop/kb/bringing-back-tools.html
I'm not sure what you are seeing with the Liquify tool. Would you be able to do a screen recording to demonstrate what happens?