George Lampman secured land from the Crown Land Agents in 1836. He became involved in a dispute over his payments, which I have copies of from the Archives. The Eaton family also secured land in the area in the same year. And, by 1840, George had married Alvira Eaton.
As to the variation in the ages for census - 57 (1861), 64 (1871), and 86 (1881). I do not find the variation insurmountable. I have found many, many, improper ages on census records - the first two are within the range, and the third is 10 years out. The key is the fact that every census, and the death record all state England as the birthplace. As to the birthdate that was placed upon the tombstone, it was probably derived from a prayer book, or Bible. Usually family pulled a record for an exact date. While I do not have any other record, it makes sense that George indicated his birthplace was in Dorset. He probably had a strong accent that made them think it was Dositshire, and not Dorsetshire. You are right though, this is my only link to Dorset. But, I would think he knew where he was born, or at least the shire. But arguments from silence, are just that...
Lampman is not a Dorset name - I have yet to figure out why he is Lampman, and not some variation like Lamson, Lamon, etc. He could write, as letters with his signature from the 1840s, and his signature from the 1838 militia paylists attest to...
Anyway, glad to have some responses.
PS not sure why you would suggest that origins on tombstones are weak sources... I have a number of ancestors who have tombstones with origins on them, and they are accurate. For example: My 4th great grandfather, Hector MacLean had his birth date (exact), and place of birth (Tiree, Scotland). This is exactly where he was from... Another, example also McLean - my 4th great uncle, Gillean McLean, is indicated to be from Inverness-shire, Scotland on his tombstone - again accurate. One more - my 5th great grandparents tombstone (William Allen and Esther Armstrong) gives exact information (date of birth, place of birth, and even year of immigration, also date of settlement in Madoc, and finally death). This also corresponds to records - including a letter of recommendation he brought with him from Connor, Antrim, Ireland. So, my own interaction with tombstone information has been good. Yes, there are mistakes on some, especially if erected by later generations, and not the immediate.