Question 1- I'm always going to err on the side of staying in a B and B, or a hotel.
As a local I'm too close to the subject, so have my own biases. But I'm very much aware of how the holiday lets and 2nd homes (lets for much of the time) have ripped the heart out of some of the villages. It's too complicated a subject for this forum, and I don't really want to guilt trip anyone, but I do feel strongly that a B and B/hotel is the better way from an ethical standpoint.
However if it was going to be a holiday let for a week then you are probably looking at a Saturday to Saturday. Sykes' gets a terrible write up on Trip Advisor- how much is people 'piling on' is impossible to know.
In the hypothetical case I was in that market then I have thought for many years that I would use a local company such as Lakeland Retreats- they are based in Ambleside (where they have an actual office) and also have an actual office in Keswick. There is just a comfort factor in that, if there is a problem, I can go and see someone physically, not just on the end of a 'phone.
Question 2- Once the schools go back at the start of September everyone can breathe a hearty sigh of relief. However what then promptly happens is that the "snowbirds" appear. So although things quieten down, but still a real busyness. While the weather is impossible to forecast September can be a glorious month- the further on in the month we are just starting the slide to autumn, so I would argue for as late in the month as you can manage.
Grasmere vs Ambleside- you are really talking degrees of relativity. Ambleside feels busier I think because of the road layout, but has the marginal benefit of being the public transport hub- it is a marginal benefit given that Grasmere has onward connections until 11 at night, but from Grasmere you can't get to Ambleside for the first Great Langdale/Coniston walkers buses at around 0630 if you want to be up and about that early.
Windermere is the same- the centre feels busy because of the road layout. But once you get off those key 2 roads it is so much quieter, yet still very accessible for public transport. So location is everything in Windermere.
For me Ambleside has the benefit of 3 cinemas/movie theatres and their associated restaurants!, but that is me.
Also don't forget that pretty much anywhere in the Lake District has two personas- the daytime trippers, and the evening when they have all gone.
Personally I'm surprised that your results are finding so little for Hawkshead and especially Coniston. For a specialist in that area try another local company- the Coppermines Lakes Cottages . Again they have an actual office in Coniston, and have a history of dealing sympathetically with the conversion of once miner's cottages (their origins) into holiday lets. In that area, they would be my go to if I was in that market, having seen the way they have grown and developed.
In Coniston you have the benefit of buses in two directions- the 505 to Ambleside (and beyond) and the X7/11/X12 etc the other way towards Ulverston and beyond (a bus route which has it's own friends club, and gives access to some great off the beaten path places), and the Coniston Launch still does a great value weekly ticket for hop on hop off lake exploration- £38 for the week last year (one round trip is £22). A launch ticket also gives discounted admission to Brantwood (buy on board) and the Ruskin Museum (at the Museum!).
The Lake District is not the Isle of Skye- booking 12 months and more ahead- I just don't see that same lack of supply. A lot of people book to come up when they see the weather forecast a few weeks ahead.
