Yes I know what you mean about the height limits being a bit buried. TBH personally, I did let my DS go over the height limit in the Advansafix that we have, because I have the one which is identical to the older, R44 one which had an 18kg limit, but no height limit on the harness. He was 109cm and still under 18kg when I moved him to seatbelt. Whether I would have felt as comfortable going over both 105cm and 18kg, I am not sure, but it didn't come up so I didn't have to worry about it.
If your DD stays on the same centiles I reckon she'll probably be around 17kg by the time she reaches 105cm, which is reasonable to me. Of course she might gain weight/height unevenly (they tend to do this). But your LO will probably be over a year old by the time she gets to this point, so if you wanted to keep her in a 5 point harness as long as she will fit, then you're likely to want both children in a 5-point-harness seat at the same time. You could if you want to look for a longer-lasting infant carrier in order to allow DD1 to use the Joie Spin for as long as possible. As I said before, the Cybex is good for this. However it does become a bit annoying to keep using an infant carrier once they get more wriggly and want to sit more upright from around 9-10 months - if you've previously used the spin from birth then you might not have been through that stage.
Although the limit of seats like the Maxi Cosi Titan or Britax Evolvafix is 105cm like the Joie i-Spin, in your scenario I don't think there is a benefit of opting for a longer-lasting harness in the 123 seat (e.g. Joie Bold). The reason is that the safety rating on those seats is not as good as the safety rating on the 105cm limit seats (the ones I mentioned specifically). Since your DD will already be older by the time she reaches even the minimum limit to move to a seatbelt, there is not sufficient benefit of the extra harness time which outweighs the lower safety rating. For someone with a child who is on higher centiles, the benefit does outweigh this downside.
The only time a seat with a larger limit would be a benefit (IMO) for you would be if you went for something rear-facing, like the Avionaut Sky.
Tension on the seatbelt I am almost certain is a myth - I think this started because when it used to be common to put children into booster seats aged 3, it was not unusual for people to consider doing so before 15kg even. That is not a good idea (because children under 15kg will be on average younger and they are definitely smaller and more vulnerable), and in order to dissuade people from doing this, some kind of myth about "the seatbelt lock won't engage" before 15kg came about. But honestly, this doesn't make sense. The seatbelt lock can be engaged simply by pulling sharply. This is nowhere near 15kg worth of force. And a lot of infant carrier car seats weigh around the 3-4kg mark but can be fitted using a seatbelt, meaning that babies would need to weigh 11-12kg before the seat would even work, and that's not the case because babies normally weigh about 3-4 kilos at birth, a six month old weighs about 7-8kg. 11-12kg is more the weight of an 18+ month old.
There are lots of good reasons not to move children to a booster seat before 15kg (and as close to 18kg or over if possible) but I don't think that seatbelt tension is one of them. If anything, it's actually the other way around - from old crash data where younger children are using seatbelts or booster seats rather than a 5-point harness, it seems that the seatbelt holds them too stiffly which is actually the cause of some of the injuries to children in that age group, and one of the reasons why a car seat is better for them.
I see the point about Britax and the marketing post they did - I do think this was probably a bit of a brand war type thing and I was a bit sceptical about it at the time. I have since learned that they did (kind of) have a point in that impact shields do have risks which aren't present in 5-point harness seats, and particularly the models of impact shield seats which were on the market at the time of that post. However they do have other benefits and overall from the data that we have today, it seems that the benefit/risks cancel each other out. So the difference statistically between (any/all) forward facing 5 point harness and an impact shield is basically none. It is still misleading (IMO) to claim that impact shield seats are dangerous in comparison to a forward facing 5 point harness. However, claims about impact shields being safer than a 5 point harness, which is how they were being marketed at the time by Cybex, Kiddy etc, are also misleading.
It's complicated and basically it's an opinion issue. Different experts in the field have different opinions about which type of restraint is best. It doesn't mean that any expert on either side is wrong or doesn't know about safety. There's no way to eliminate every kind of risk and each of those types have different approaches and different risks they are trying to eliminate. The newer legislation also has changes to the requirements about impact shields so may address some of the issues with them (but honestly, I don't know if it helps or not.)