Is passenger information (as entered during online check-in) other than name and visa ever validated by airlines?
For US/UK flights the information you entered during online check-in seem to be always thrown away. What seem to happen in practice nowadays is that your passport is always scanned/swiped during the “document check” (which happens either during check-in, or at the gate) and the information you have entered during online checkin is overwritten. The gate agent explained it was faster for them to do it this way than cross-check the data and correct mistakes. You can watch it yourself when they do it.
Is that information ever validated by immigration?
It depends on how you define “validated” (I assume we’re mostly concerned about the practical aspect, i.e. “what happens if the airline transmitted the incorrect information I entered, and it doesn’t match my documents?)”. However some of us routinely travel with two passports, and usually the information from only one of them is transmitted. For example, I routinely fly to Istanbul showing only the US passport to the airline, but enter Turkey using a different passport (which the airline knows nothing about) – never had any issues.
Even if they did, you can’t be blamed for this because you did not transmit this information to immigration yourself. More, the law puts this responsibility (and liability) on airlines, not on you. If they let the inaccurate information through, it is their fault – and their fine.
Update: last month I have requested (FOIA) the CBP arrival records for my Mom, and they had inaccuracies. In one case she missed the flight and took a later flight 7 days late, but CBP had record of her arrival in USA on the date of the missed flight – and this arrival was NOT recorded in CBP database. Thus it is pretty evident that this information is not checked at least by US immigration.
Will you be denied on-board because you’ve made a mistake during
check-in?
Every time I get a new passport, when I do online checkin, I typically make mistakes in passport number and the expiration day. It has never been a problem, and I was never denied boarding. This included multiple flights to USA/UK.
Final: since the data you entered during online checkin is always overwritten during the document check, it is impossible to be denied boarding for the mistakes/typos made during online checkin*. Even if for some reason the airline decides to verify it, they would simply enter the correct the data and let you through. As mentioned in previous version of this answer, I have done this in 2016, and I have done it a lot in 2017 (being pissed off that airline requires you to enter a shitload of data which they throw away anyway).
(*) of course you can still be denied boarding for other reasons, like bringing a wrong passport. But this was not the scope of the questions.