Never assume that the only candidate in the records is the right one.
Before the start of civil registration in 1855, large numbers of baptisms and marriages, and most deaths, either were not recorded or if they were the record has not survived.
George Herbertson, son of James H and Janet Dougal, was baptised in June 1807 in Glasgow, four months after the baptism of Robert H in Dalton in 1807. Therefore if this George is the one who escorted Alice to Australia and then returned to Scotland, your Robert isn't the one born in Dalton.
George Herbertson, mother's surname Dougall, died in Glasgow in 1886 aged 78.
There was also a John born to James H and Janet Dougal, baptised in Glasgow in 1805.
You say that Andrew and Alexander both arrived in Australia after 1851.
In 1851 there is an Alexander Herbertson, 28, wright, born Glasgow, living in Cumbrae, Bute with wife Catherine and children Margaret and Andrew. Cumbrae is where James H died in 1858.
Who is named as James H's wife in his will? Who was the informant on his death certificate? According to the 1851 census, he was born in Dumfries. There are three baptisms of Herbertsons with mother's maiden name Linton in Dalton, Dumfries-shire: Janet 1773, Rachel 1777, and Thomas 1779.
In 1841 Henry Herbertson, clerk, and Alex Herbertson, apprentice wright, are living together in Glasgow. Both ages are given as 15, but in 1841 adults' ages were supposed to be rounded down to the nearest 5 years, so they could have been any age from 15 to 19, and therefore (if the census age is accurate) born between 1821 and 1826, which does fit.
Henry Herbertson, mother's maiden name Hunter, died in Glasgow in 1876. Have you got his death certificate?
I don't see the baptisms of Alexander 1823 or Henry 1825 in the indexes at Scotland's People. Where did this information come from?
There's a household in Cumbrae, Bute in 1841 consisting of James Harbertson, wright, 53; Margaret H, 50; Mary, 13; William, 11; Andrew, 9; David, 4 and Margaret under 1 year. Margaret Sr and William born in Bute, the rest elsewhere in Scotland. The only one in the baptism registers is David, baptised 1836 in Gorbals (Glasgow).