My wife arrived into Heahrow on Tuesday.
The people shuffling agents said no need to get a stamp, not satisfied she talked to a group of border agents just milling around and they said egate is fine even if it's the 1st time and no stamp needed.
The point we are trying to make here is that although they are telling people no stamp is needed, they are not showing where this is written in the immigration rules, nor are they advising how people are going to prove their date of entry for all of their next visas down the line.
I'm encouraging people to push back and either insist on a stamp or for written evidence of how it's going to work when it's time to apply for ILR, which I am pretty sure they won't be able to supply.
Meanwhile, in your situation, keep all the concrete proof you have of her date of entry.