How do I completely uninstall CleanMyMac and MacAfee security? They are causing my iMac to be unstable and crash frequently.

Both should have uninstallers either separately or build into them. However, you can check to see if you've removed all of the supporting files by downloading and running the shareware app Find Any File to search for any files with the application's or the developer's name in the file name.  For those two apps you'd do the following search(es): 

1 - Name contains cleanmymac

2 - Name contains macpaw

3 - Name contains macaque

Any files that are found can be dragged from the search results window to the Desktop or Trash bin in the Dock for deletion.

FAF can search areas that Spotlight can't like invisible folders, system folders and packages.  

If you get warnings that the file can't be deleted because it is in use or used by another app boot into Safe Mode according to How to use safe mode on your Mac and delete from there.

Note:  if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode.  This makes it act as a wired keyboard as will assure a successful boot into Safe Mode.

Apple support tells me I may have to do piece meal reinstalls from my time machine files. I will have to study up on that. It sounds like an extremely arduous and time consuming process.

You're both right... it is an extremely arduous and time consuming process.

The first thing I do with any Mac that has ever been affected by any non-Apple "cleaning" product is to erase it. It saves time and aggravation — both of which are mine to save. However, if you are so motivated and have sufficient time to explore options short of that, review the following. Some of it may need to be adapted to the particular macOS version you are using, and if you have questions regarding that please write back.

As a category, non-Apple "cleaning" products are scams. Merely uninstalling them, even when performed in strict accordance with their instructions (which often don't work) is usually an insufficient remedy due to the effects of having used them.

Very briefly stated, the recovery procedure from having used such things is as follows, and in the following order:

1. Back up the Mac using Time Machine or an equivalent backup strategy
2. Remove the scam "cleaning" products according to their instructions, and evaluate
3. Reinstall macOS using macOS Recovery, and evaluate
4. Reinstall all other potentially affected apps, and evaluate
5. Erase the Mac completely, followed by reinstalling macOS, followed by reinstalling essential apps from their original sources, followed by migrating User content from the Time Machine backup.

Since the results of Steps 2-4 are almost always unsatisfactory, I omit them as unnecessarily time-consuming.

An expanded procedure for Step 5 is provided below. Be sure to read everything and understand its implications. Less drastic solutions certainly exist, but this one will probably result in the least amount of your time and need for constant interaction. It is also unequivocally supported by Apple's published documentation, is what a Genius Bar would do if you were to pay them to do it, and it is the exact same procedure I use given a Mac that has been affected by similarly categorized products.

To recover from the effects of having used "CleanMyMac" to modify your Mac, the additional software you require and the essential files you need, follow the applicable recovery procedure below:

• If you have a backup that you created prior to using CleanMyMac, now is the time to use it. For Time Machine, boot macOS Recovery, and at the Mac OS X / macOS Utilities screen, choose Restore from Time Machine Backup. Choose a date preceding the installation of CleanMyMac.
• If you do not have a backup that predates the use of CleanMyMac, create one now. To do that read Use Time Machine to back up or restore your Mac.
  • The recovery procedure will require that you completely erase the Mac using macOS Recovery, and then create a new User Account whose contents will be empty. You will then be able to use Setup Assistant to migrate your essential documents including photos, music, work products and other essential files.
• To erase and install OS X / macOS read How to reinstall macOS.
  • Follow the instructions in that document under use Disk Utility to erase your built-in hard disk.
  • Then, follow the procedure in Move your content to a new Mac.
  • When asked how you want to transfer your information, select Transfer from a Mac, Time Machine backup, or startup disk.
  • Under Select the Information to Transfer, select only your previous User account and do not select "Applications", "Computer and Network Settings" or "Other files and folders". De-select those choices.
  • Subsequent to using Setup Assistant, you will need to reinstall the essential software you may require, once again remembering to install software only from their original sources, and omitting all non-essential software.
  • "Non-essential software" is a broad category that includes but is not limited to third party "cleaning", "maintenance", and "anti-virus" products.

Post the EtreCheck report. Follow these instructions: How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report - Apple Community

The EtreCheck report will reveal any remaining components, but be prepared to erase that Mac completely. Merely uninstalling scam "cleaning" products, even when performed in accordance with their instructions, is insufficient to reverse the damage they are capable of inflicting.

Create a Time Machine backup if you haven't already: Back up your files with Time Machine on Mac - Apple Support. Creating a backup will be an absolute prerequisite for additional troubleshooting, so you might as well do that now.