I finally ordered replacement parts for my trimmer and received them a couple of days ago. On Tuesday night I sat down, disassembled the unit and replaced the switch. It turned out to be about a 30 minute job (Went slow and "measured twice and cut once" as the saying goes). I did not consider it difficult. Although I ordered the switch, trigger and spring (Part numbers 6, 7 and 8 on the parts explosion diagram) I decided to replace only the switch. I took it outdoors next day, started it up and trimmed enough grass and weeds to allow me to declare the repair a success.
Based on my past failure rate I ordered three of each part, so I should be covered for the next three years. From a practical point of view the switch appears to be the weak point but the other two parts were inexpensive and I'll have them just in case. The new switches had a different manufacturer's part number stamped on them, hopefully denoting a 'new and improved' version. I have no idea how long these switches will be available through Ryobi channels but hopefully they are an off-the-shelf electronic item.
I would like to thank Jim and UtahAV for words of encouragement on this repair and apologize to joeblough for not doing this sooner so I could answer his questions. I'm not rude, just slow.