That's tough. I haven't had to deal with this yet for Jackson but I know I will one day. And I DO feel guilty already on the rare occasions that he's left alone for 8-10 hours. So I imagine 5 days a week is going to make me feel guilty. But honestly, he's always fine. A bit more needy, and needing some trick training or play time, but otherwise? fine.
But he hates strangers coming into our home without me and I don't think he would accept a dog walker in his house and leashing him up, he also wouldn't be happy in a doggy daycare environment without me, sooo... I know he would end up being happier at home even if it sucks and it's boring.
But if you think Merlin would be okay with a dog walker coming in, which it sounds like he would be, I would probably opt to do it at least a few times a week. I don't think you will mess him up too bad if it's not every single day.