To be honest while Welsh isn't my first language (because we're close to the border) in school we seemed to have learned the south dialect. So despite the fact I live in the North I've always used eisiau and I've actually never heard isio until now. Same with all the other words here.
In the 2011 census, the areas with the highest numbers (not %) of Welsh speakers were, in order: Carmarthenshire, Gwynedd, Anglesey, Cardiff, Ceredigion, Conwy, Rhondda Cynon Taf, Swansea. So, a mix of areas, really, and spread out across the 4-6 (not 2!) main dialects.
Official documents, etc are not written in a particular dialect.
Definitely not true!
Novels that I have read recently have used a mix of all sorts of dialects: Kate Roberts' famous short stories use spoken dialect from the quarry area between Caernarfon and Porthmadog, Alun Jones' novels ('Ac yna Clywodd Sŵn y Môr' is a GCSE set book) have a lot of Llŷn dialect in them, Bethan Gwanas uses a lot of NW Wales dialect in her books, 'Veritas' by Mari Lisa uses a lot of mid-Wales dialect in the quoted speech portions and characters from elsewhere seem to use their own local dialect, T Rowland Hughes used a lot of north Wales quarry district dialect (in 'O Law i Law', for example), 'Y Llyfrgell' by Fflur Dafydd is set in Aberystwyth, ....
Mainly because the Welsh Assembly is in Cardiff and the south just like to pretend that the north doesn't exist... But you know that's probably just me being biased.