I'm sorry that you are in this tricky situation OP.
I'm primarily a dog walker, but also do some sitting so have some experience from "the other side".
It sounds like posters have had good experiences from agencies like Rover and Trusted Housesitters. I think, like almost things in life it really depends on the individual that you get. It's worth bearing in mind that they are agencies so it's really important that you know exactly who you are getting etc..
I rarely do sitting over periods of more than one or two days. Mainly because I need to manage the logistics of my own family, pets and dog walking clients alongside caring for someone else's dog in their home.
Every sitting client that I take on I visit first. Depending on the dog I'll also do a trial and for a sit of the length of time you need I'd also do an overnight trial. It might take some time for a nervous or reactive dog to feel comfortable with me, and it is important that this isn't rushed so I may need to visit a number of times, and work with the owner to build up trust with the dog and learn how best to manage their challenges. I would need to be paid if this goes beyond the initial visit and shorter trial.
I think a good and reliable sitter will not over-promise eg until they have met your dog and done trial(s) they will not confirm a booking. They will know their limits and they will ask plenty of questions to ascertain whether a particular dog can be cared for with their own skill set.
A good sitter will discuss exactly how long your dog can be left alone and will have a plan for achieving that. They must be able to achieve that whilst meeting other commitments (so in my case feeding my teenagers, caring for my dog, working for my dog walking clients etc..).
They will also discuss contingencies for things like crazily hot weather, emergencies, vet visits etc..
I do try as much as I can to work with whatever the dog is used to at home (with the caveat that I work in a force free, reward based way so no aversive equipment, punishment etc).. The only time this tends to be tricky is having the dog sleep on my bed at night. I can do this, I love a cuddle and snoring dog- but it does depend on the dog, how well we bond etc.
Behaviour wise I know my limits. I'm fine with over enthusiasm, low to medium challenges like separation anxiety, barking, resource guarding etc.. I'm cautious with reactive dogs, unpredictable dogs and very fearful dogs because in my experience the most worrying behaviour comes from fear and working on this takes a great deal of time and flexibility which os tricky is you're in a situation of being in sole charge of a dog for a fixed extended period.
Hope that helps.
I'd suggest that key elements of care are put in writing: dates, agreement about alone time, important elements of care etc. to avoid misunderstandings.
Hope this helps a bit.