I don't know whether that version of the BabyAuto Dupla needs a top tether. Realistically, the only thing you can do is order it and then check the manual. Or maybe see if you can ask BabyAuto, or the retailer.
As far as 25kg forward facing seats go, I do trust it (the top tether version) and he would defo be better off harnessed than in a booster seat after 18kg, and at least it can rear face until he's about 2.5, so yes I would go for it.
My only reservation is the question mark over the top tether, because there is no belt fitted only version on their website, so I'm unsure as to whether it's a mistake on the shop. The other thing is that what we find is car seats up to 25kg harnessed really do need some kind of third point of anchorage, whether it's top tether holding it back from the top, or a support leg going down to the floor. (Or the Swedish tethers for rear facing seats.) If that's really belt fitted without either, I would have serious concerns about how it would actually perform with an older child. However, if it's your only option you might have to go with it. At least you know it's passed the legal crash tests with a 23kg dummy in. But I'd probably swap it to a booster seat as soon as you feel he's safe and he gets a good belt fit like that.
I wouldn't normally suggest second hand, but you could have a scout around to see if you can find any second hand 25kg ERF seats, if there's no top tether available and you want him rear facing. At least someone who's gone to the trouble of buying and using an ERF seat likely knows the importance of not selling on after an accident.
The thing with the sizing of the Tilt is that they are marketed and labelled everywhere as "Up to 4 years" which is probably why she thinks you're being daft. I would point out that he's the size of a 3yo at 1.5 / not yet 2, so while the seat she's picked will fit an average child to age 4, if your child is the size of a 4yo at age 2.5, then it will be too small for him. That's why you want a seat which goes up to age 6, because that's likely to last him until he's about 4. And their weight gain levels out, so he won't be massively above his age forever!
It's only a marketing thing, because if they list it by weight, people have no concept of what that actually means, at least years gives you a rough idea, and it's accurate for about 60-80% of children. The problem is, anyone with children in the top or bottom 10-20% is going to find that their kids are way outside these estimates, it's just like clothing sizes really. Maybe if you explain it to her like that she'll see what you mean.