I think the first think to do is sort out some kind of alternative care - can MIL continue to pop in to see to dog? Can you start getting recommendations for good local dog walkers who understand the need of growing pups? Do any of your local kennels offer doggy daycare.
Although you'll get all the people coming on saying "my dog is locked in a coal cellar for 18 hours a day and loves it", leaving a dog, especially a young dog, alone for the length of time you're proposing is not ideal. Labs are intelligent dogs who need mental stimulation and human company. They also benefit from having their exercise built up slowly while they are growing to minimise joint problems in later life, so don't assume you will be able to tire out your pup with long morning walks from the point it's had its jabs in order to feel less guilty about leaving it all day because "it's had a good walk". Yes it will need plenty of walks, but carefully monitored with the amount of free running / high impact exercise restricted at first and increased gradually.
They are also renowned for being a destructive breed if left to their own devices for too long and while I am an advocate of crate training, it isn't acceptable to lock a dog in a crate for that length of time especially if alone in the house.
TBH I doubt any really ethical breeder would consider you as a suitable home for one of their pups unless you could demonstrate at the point you first make contact that you have plans for daycare in place. Otherwise you'd be restricted to the BYBs who are at best ignorant and at worst unscrupulous, or puppy farms.
If you can sort out someone to look after or at least pop in on the dog through the day, I'd suggest looking for (and you won't have to look very far, sadly) a rescue Staffy - and I say that as a retriever specialist. Staffies make amazing family dogs - great with children, real go-anywhere dogs with an unfortunately misunderstood reputation. A reputable rescue will match you with a suitable dog for your needs.