Comfortis is great stuff but super expensive compared to all else. Apparently they're now also making a new product beginning with "P" Proforrtis or something - a broad spectrum that does fleas, worms x 4 and HEARTWORM, so basically a Sentinel but different company. If the price is comparable I'll go between the 2 each month as I prefer to rotate brands to prevent immunity. If horse worms can become immune to one product, then I'm darn sure dog & cat ones can too.
I wouldn't expect you to see any fleas or flea dirt on the floors and yes, it is a very good sign you didn't see any on Toby, it means they did have a chance to suck his blood & poop for days. It usually falls out of certain coat types too until there's a real disgusting build-up ime.
I'd say that based on the fact you brush him every day and are therefore going to see anything untoward as soon as it happens that you've caught it really early.
All this vacuuming and hours of washing & other back-breaking work .... please, please remember this - what do you use when you go fishing? Yes! Bait, the fish will go after the worm, the shrimp or whatever it's food is! Ditto fleas - think of your dog as the flea food, he's your bait ... a horrid thought but there's no other way to describe it.
You didn't need to do all that work because as soon as those fleas are capable, they rush to your dog in their droves, get on him and are killed either on contact or when they bite (depending on which product/s you've used).
It's a natural ongoing & neverending cycle I'm sorry to say, and it's precisely why, in flea infested areas, you need to keep up the flea products 365 days if you really want to be as free flea as is possible - they will ALWAYS pick up stray fleas the second they walk out the door & bring them back inside - it's what's on the "bait's coat or in their bloodstream" that dictates how long that flea will survive, whether it will be just sterilised (Sentinel) or in fact killed outright.