Tom, as you know doubt realise, I've ploughed quite a furrow from LM to Calais and back in the last few years! I've tried going via Paris, I've tried rural routes, I've tried going via the Pont de Normandie and Le Havre, and in spite of this, I've never found a quicker route than via Rouen. Timing is all. Avoid the rush hour like the plague ie. don't get there between 8 and 9 in the morning or between 5 and 7 in the week (4 and 6 on poets' day.) I realise that for LM you're restricted because of crossing times but better to just stop and have a long relaxed lunch or whatever to get your timing right than get there when the locals are going home.
I don't know about mapquest, as I've always used the old paper kind myself, being an old-fashioned gal! However, maybe it's worth your entering say "Reading to Rouen" then "Rouen to LM" - to force it to give you the info you want?