To answer the various questions in no particular order...
The Chesham service is diverted to Amersham as detailed below.
TTN 99/15 said:
Commencing with Train 405, 10.02 ex Aldgate up to and including Train 403, 15.02 ex Aldgate , all trains booked to run to Chesham will be diverted to Amersham to detrain in platform 2, depart to siding at xxj151⁄2 or xxj451⁄2 and form xxb311⁄2 or xxb011⁄2. Cancelled Chalfont & Latimer to Chesham.
Commencing with Train 405, 11.26 ex Chesham up to and including Train 403, 16.26 ex Chesham, all trains booked to start from Chesham will instead start from Amersham (ex siding) to platform 3 depart xxb011⁄2 or xxb311⁄2 and pick up their normal path at Chalfont & Latimer at xx.05 or xx.35. Cancelled Chalfont & Latimer to Chesham.
A replacement bus service will run between Chesham and Amersham every 15 minutes throughout, with a temporary stop being located outside Chesham station.
All services via the North Curve as it makes pathing easier up from Harrow - i.e. straight up the Main.
Chesham station - I understand that people will be allowed on the platforms for photos but that will be at the discretion of station staff on the day. The train will be operating a ticket+wristband system to actually be able to get on board. To view the train at any other station a valid LU ticket or Oyster card is required.
Someone else has already posted the link and prices for tickets, but I can confirm that there are still some available for sale, and there may be some available on the day.
Finally - a word of warning. Last year there was an awful lot of incursion onto platform ramps, the track and other non-public areas. Lots of tripods and flash photography was also being used, as well as photographers straying over the yellow line. Please, please, please obey the TfL conditions of carriage; none of these are okay, and they will jeopardise future vintage events on LU. Additional BTP officers and SRT staff will be deployed on platforms this year to help manage this.
Oh - and the formation should have Met 1 and Sarah on the north end and L150 and the milk van on the south - but all subject to change until it leaves Ruislip!