during your trysts, so she will think you are working out.  (You are, after all, aren't you?)

 
Get a burner phone to use for lobbying, and find a safe place to store that so she won't find it.   The oven is usually a pretty good place.  8o)

You can spoof the phone number and text to reflect your phone number through APPS and online services; Spoofbox, Spoofcard, and spoofmyphone.

If you have, or can get, another iPhone or an iPad or even Apple Watch, you can choose which device to set as the device whose location you share.  Viewers don't get told which device it is, only "your" location.  Then put that device in a neutral place while going elsewhere.  
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Tracfone offers older iPhones -- a bit pricey, but maybe what's needed for your case if you don't already have another Apple device you can use in this way.  Or buy an old iPhone on eBay (check how old it can be and still serve as a FindMy device; I don't know the cutoff).  
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This approach does not spoof the location of a device, it just sends the actual location of a different device set aside for that use.
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Just don't forget to change back to the known iPhone before you return!  That would get you busted.
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FindMy is completely separate from phone calls, so forwarding is not an issue that affects location.  BUT- You should keep your burner phone completely separate from your regular life.  Never have your burner number on your regular phone in any way (Contacts, web history, Notes, anything).  Never have your regular phone number on your burner (ditto).  
 Forwarding calls between them is a big bad vulnerability (as you note).