The comments at the time was that some of entries were overstatement and were interepreted by some as understanding a few words (see http://www.ee.surrey.ac.uk/Contrib/manx/mannin/v2p080.htm for the 1911 comments and a rapid decline) - all other indications (see eg Jenner's survey of 1875 (see http://www.ee.surrey.ac.uk/Contrib/manx/catalog/h140.htm), his quoted comments from the Vicars, the comments re founding of Manx Lanaguage Soc, Methodist LP's who preached in Manx were already in middle age by 1850 and few young ones came up after them). The 1820 date was that estimated by a philologist who used changes in pronunciation to indicate that by 1820 most learners of Manx were doing so in an English speaking environment, others have argued that 1840 was possibly the correct date - but c. 1820/30 agrees with my reading of Methodist LP's who kept the language going longer than others. Yes speakers were still alive (the last native speaker, Ned Madrell died in late 1960's I think - many have said that a language dies much more slowly than you think as there are always isolated pockets eg Ned was brought up by Grandparents in isolated Cregneish - but by 1920 the language was virtually an antiquarian pastime - look at members of YCG.
It continued to have influence on Anglo-Manx dialect - just as Irish gaelic did on English as spoken by the Irish but this is not the same thing.