3) On the off peak travelcards, the railcard rate for zones 1-9 + Watford Jn is £8.90. For zones 2-6 it is £5, the same as zones 1-6. For zones 1-9 and 2-9 a railcard discount is available, but the price is not published anywhere. I think that BR ticket offices are unable to sell these, therefore you have to use a LUL ticket office.
The Zones 1-9 Travelcard is available from BR Ticket Offices.
It is issued from an "Origin Station" like for example a Watford Junction Travelcard. The list of origin stations are Amersham, Chesham, Chalfont & Latimer, Chorleywood, Croxley, Rickmansworth, Watford Met. These are the only London Underground stations that can be used as 'Origin' stations when issuing Travelcards.
Obviously stations on the branch from Harrow & Wealdstone to Watford High Street can still purchase Travelcards into London as they have always been able to.
The routeing of all the above will be AAA ZONES R789 designating they are valid for travel in Zones 1-6 plus Zones 7,8 and 9.
The only Anytime Day Travelcard prices for Zones 1-9 (ineligible for Railcard discounts) are £16.20 Adult, £8·10 Child.
The undiscounted prices for an Off Peak Day Travelcard Zones 1-9 are £9.00 Adult, £2.60 Child. The 16-25 Railcard price is £5.95.
Zones 2-9 Travelcards are also issued by BR ticket offices.
The Anytime Day Travelcard prices for Zones 2-9 are £10.60 Adult, £5.30 Child
The undiscounted prices for an Off Peak Day Travelcard Zones 2-9 are £5.60 Adult, £2.40 Child. The Zones 2-9 Travelcard is INELIGIBLE for Railcard discount, contrary to what I have posted above, stating it is infact valid for all the Travelcards the OP listed. The Railcard discount is only valid for All Zones travelcards (of which Zones 1-9 is also included).
One suplementary question from me. As Watford Jn is effectively a travelcard zone all of its own, is a Watford Jn-Zone R1256 valid for travel from central London to Watford Jn, and back in from Watford Jn, or would you have to purchase the zones 1-9 + Watford Jn ticket from a LUL booking office?
If you wish to travel from Central London to Watford Junction and then back in from Watford Junction, you will need to purchase a Zones 1-6 Travelcard (either BR/LUL station) and then a Boundary Zone 6 extension to Watford Junction (which will be routed AAA ZONES R789)
Boundary Zone 9 extensions do not seem to exist and it isn't clear what occurs with someone already with a Zones 1-9 LUL Travelcard, who then wishes to extend their ticket to Watford Junction, for example.
Also, is there any reason why travelcards for the non-GLC zones aren't issued by BR booking offices (except, presumably for those actually in those zones)?
They are issued, see above.