Can I dramatically simplify the Apple Watch Gen 4 Interface?

Also, if you want to use a watch face that has complications (or more complications then you want to populate), you can set each complication field to "Off" so that it will not have anything it it.

So you can choose any watch face you want, even if it supports lots of complications, by just making turning off any complication fields you do not want to see anything in, including all the complications.

Hear are the various watch faces

https://support.apple.com/guide/watch/faces-and-features-apd6ce85daf4/watchos

That is the generally the first things your Mom will see. You can choose a very simple face, that does not include complications, until such a time as you and her decide there is something she wants to have fast access to.

I would suggest you ALSO get an Apple Watch so you can learn the Apple Watch and be knowledgeable when she needs help.

You do not need to get a cellular version for yourself, as you can do just about everything with a GPS only as long as your supported iPhone is nearby.

Getting Cellular on your Mom's Apple Watch does require that her cell carrier supports the Apple Watch. Again, if your Mom has here iPhone nearby (either in bluetooth range, or on the same WiFi network, such as at home), she can get away with a GPS only model as well.

You may also find this webpage useful whether GPS only or cellular model Apple Watch is obtained

https://support.apple.com/en-us/HT205547

There are several plan Watch faces. Use a photo. If you not sure visit the Apple Store and look and ask questions. Remember if purchased one you can take it back and get your money before 14 days are up. Ta