These days (faster vessels) the day sailing is an average of 5 hours allowing for weather variations.
They slow down on night sailings to give more time in bed.
Last day sailing I made left Roscoff at 16.30 local and arrived Millbay Docks 21.30 GMT.
Night sailing is at 23.00 and arrives 06.00-ish. As I said earlier, the night crossing allows enough time to have a drink and maybe a bite to eat on board, then get a decent enough sleep.
Personally, I don't like the area around Morlaix, it's not particularly attractive. I usually have a quick blast along the N12, turn off it onto the N176 for Dinan. From Dinan the countryside gets much nicer, Mont-St-Michel nearby etc.
If you don't have accom planned, whenever you get to somewhere that you might want to stop, go to the local Tourist Info Office (well sign-posted) and you will find helpful staff. They will give you lots of info about local hotels, directions and leaflets.
Again a personal preference, it's better not to tie yourself down to being at a certain place at a certain time because of pre-booked accom. France (like most of Europe) is far better served by plenty of small, inexpensive but excellent family-run hotels. It's easy to find somewhere each day if you want the freedom to just suck it and see.