It's unlikely that it would invalidate your insurance, but if the handbook says no, then there will be a reason why it's not a good idea to put a car seat there. It might be that you can't disable the airbag for example, airbags can be dangerous even for forward facing children.
Is it a Zafira or Zafira Tourer that you have? (Not sure if I'm looking at the right manual). And is it a seven seater - could you not put the HBB in the very back row?
What I'm seeing (I've looked at the Jan 2014 manuals for each version, which might not be correct)
Zafira (not Tourer) says:
No child seat of any type in front seats
Any child seat allowed in centre second row seats, but no isofix available.
Any forward facing (group 1 or 2/3) seat in third row, but no isofix points here.
Then the Zafira Tourer says:
Group 2/3 seats not allowed in front seat
BUT you CAN put a Group 1 (9-18kg) in the front seat with airbag disabled.
No child seats allowed on centre seat second row, at all.
Any belt fitted seat for any weight class in third row seats.
So I'm thinking, if you have the Zafira, it's a no go in the front but you could look at slimmer HBB seats to fit between the other two and/or look at replacing one or both isofix seats with a slimmer and/or belt fitted one to give you more (literal) wiggle room. Or put eldest DD in very back row in the HBB that you have.
And if you have the Zafira Tourer, then you could put the harness back in the Elevate and use it in Group 1 mode in the front until DD hits 18kg (which seems like it will be a few years). Or again put 6yo in the boot seats.
Legally, you're allowed to carry the eldest child without a car seat if you can't fit three in a row, but I wouldn't recommend doing that as the seatbelt is unlikely to fit her well at all at 16kg - it would be quite risky. (Also I don't know if it applies to 7 seater cars).