I agree with Toki and Carol... Sergio's behavior with this woman was perfectly appropriate. SOMETHING about HER made him back off. I wonder whether part of the reason he didn't warm up to her was that, because SHE was lecturing you, maybe YOU started to give off vibes that you didn't care for her. (I would have agreed with BOTH you and Sergio!
Kodi is about as well socialized as a dog can be. I have worked on it extensively from the day I got him, and the Kings worked on it extensively while he was still with them. I don't expect him to go leaping into the arms of every stranger who comes to our door... in fact, I am happy that he is our little "alarm dog" and lets me know when someone is on the property. He stops barking when I ask him to, and invite the person into the house, but he certainly doesn't leap into their arms. If they give him a few minutes he will warm up, and if I give them a treat, he will come to them. But then he would wander off, either to my side, or back into his crate. (Ooohhh, this woman would probably see that as EXTREMELY antisocial!
OTOH, once he has met a person a couple of times, if they are friendly to him, he is a friend for life. We have been going through this renovation to our house for 9 months, with various workmen on and off the property almost daily. After his initial letting me know someone is there, he greats each of these people (all of whom are familiar by now) as friends.
I wonder if this dog walking service is planning to have a different person come to get him at different times, and that is why they re so concerned that he go to anyone, any time. (I don't know if I'd be happy with a bunch of strangers entering my house!) If it's just one person who will be walking him, perhaps you could arrange and pay for a few half hour meet and greets, first with you there, and then with you leaving so that the person can go in and interact with him on their own.
I suspect he'll be fine! After all, you aren't his original owner... he met you as an adult, has adjusted to you beautifully, and you haven't had to worry about him biting you!!!
As far as the walker who picked him up at the groomers, I, personally, wouldn't let someone who didn't already know Kodi VERY well pick him up. NOT because I think he might bite them, because I am quite sure he wouldn't But because he might struggle to get away and end up getting dropped and hurt. Unlike cats, dogs don't do well getting dropped from arm-level height. (and I don't think people should pick up cats they don't know well either!)
Also, it's just not something I think he should have to tolerate from strangers. Even at the vet's office they don't do this. They have the owner put the dog up on the table for examination. It's a REAL invasion of the dog's personal space.
Finally, as far as using treats for training is concerned, it isn't an age thing. If you were working with a 10 year old untrained dog, you'd probably need even MORE treats than when training a puppy. But treats are, for the most part, for training new and difficult behaviors. You want to fade treats for established, "expected" behaviors. For instance, no matter what the dog's age, you would probably use quite a few treats teaching them loose leash walking. However, once they have been successfully walking on a loose leash very reliably, you would fade and then totally stop giving treats for it. At that point, if they dog began to pull, (let's say they saw a squirrel run across the path) you would simply stop walking, stand still and wait for the dog to return to your side. (negative punishment... taking away the pleasure of moving forward)
So, you are right, you don't use treats forever, UNLESS you want to challenge your dog and keep them learning new behaviors.. Even then, the treats are for NEW behaviors, not established ones. (The only exception to this, for most people, is the recall in the open. This one is SO important, and it is SO easy for the dog to receive reinforcement for the wrong behavior in the environment itself, that it pays to keep putting "money in the bank" and giving food rewards for prompt recalls OFTEN)