If you feel the need to question the mechanic, by all means do so. My father ran his own shop for +40 years in the Bay Area and would always try to explain how and why whenever a customer had questions regarding his diagnosis and fix.
Did the Check Engine light come on? If it did, did the mechanic read the trouble codes that were set (if any)? If it indeed needs plugs and a coil, did he tell you why these things needed replacing? From your description, it sounds like he found a bad coil (the engine has eight coils, one for each plug). If the coil was bad, it's plug probably fouled. In these cases, it's pretty much SOP to change all the plugs and not just the one.
When was the last time the plugs were changed before the rough running on the freeway (replacement interval is 90k miles)? Plugs can go bad but these days they usually go bad because something caused them to go bad.
It may also be a fuel issue. I doubt fuel goes bad in a year, but it could be a "stale" fuel problem. Was the tank refilled recently? If not, remove the fill cap and give the filler neck a whiff. When gasoline goes bad it begins to smell like turpentine.