OK - so good news.
The seat can be used rear facing up to 18kg - that's a great thing for a seat in this budget range. Indeed it MUST be used rear facing up to 15kg.
The top tether is the kind where it loops all the way around. That is actually the style used in Australia where they have very good car seat safety (though alarming early forward facing laws!)
It also has a little clip on the side, so you can easily open and close it, so that you can rotate the seat without sacrificing the top tether.
The instructions aren't bad. They mostly make sense and are easy enough to understand. I would clarify a couple of things -
They don't really explain properly what a top tether anchor is. You will only have one of these in your car per isofix seating point. If your car is 2013 or newer, it will be marked with an "anchor and child seat" symbol. If it's older, you'll have to check your car manual to see if you have top tether points.
Changing the height of the straps isn't well explained. It is a fairly straightforward process but it's a more old-fashioned method these days. You want to look at the plastic shell of the seat and notice there are several horizontal or sometimes slightly angled slots for the shoulder straps to go through. Then rest the seat against a wall in the house, sit your child into the seat, (with the insert, if he is under 13kg) and note with your finger which slots his shoulders are closest to. If he's between two slots, you want the slightly lower one for rear facing. If the straps aren't already threaded through this slot, then you'll need to unhook them from the metal piece and slot them through the slot they are currently in, then move them to the appropriate slot and re-attach to the metal piece making sure nothing is twisted and you've also gone through the cover at the right place.
The seat does NOT have a forward facing mode with 5 point harness. It's unusual in its category for this (but it is a safety benefit so actually a plus). Once your child weighs over 18kg or his shoulders are more than 2cm higher than the top slot for the shoulder straps, he would have to stop using the rear facing mode and convert the seat to high back booster mode. That might be a concern depending on how high the highest slot is. Something to check when it arrives, perhaps. But it should certainly work well enough for now.
You would think the FB groups would love it because it doesn't have a forward facing harness mode. I actually can't see much to complain about with this seat. Yes, it's basic, and it's cheaper than some others in its category, but basic doesn't matter if you are rear facing. I don't think you would realistically gain anything by swapping this for (e.g.) a Graco Extend R129/Graco Slimfit, aside from the newer side impact protection - and you would lose the spin function.