I'd suggest you take a look at 'The History of the Lapp Language' and 'The Lapp Language' by M. Korhonen in The Uralic Languages: Description, History and Foreign Influences, ed. by D. Sinor (1988) and 'Saamic' by P. Sammallahti in The Uralic Languages, ed. by D. Abondolo (1998).
I read in 'Sami Language at Home and at School' by A. Jansson that she had no problem talking in Northern Sámi with Inari Sámi speakers because the languages are "with some effort mutually intelligible". She also says that while North and South Sámi share "lexical similarities" they are not mutually intelligible. Likewise speakers of North and Skolt Sámi can not understand each other.
As for Finnish, the Baltic-Finnic languages split from the Saamic languages some 2-3,000 years ago so, even despite the geographical proximity of the Inari, North and Skolt Sámi languages to the Finnish dialects, there is no mutual intelligibility at all. Sure, they share some words, but put a monolingual speaker of Finnish and a monolingual speaker of a Sámi language together and you won't observe any meaningful discussion.