I really like and have owned all the boards you have discussed. They all bring different things to the table.
Float is a consideration, but not the only one. There is no need to go to the lowest possible volume just because you can.
At your weight the 10' Mana and the 9'5 Allwave will be about the same... float and stability wise.
I actually think that the 10' Mana surfs better in small waves than the 9'6 Allwave.
The 9'5 Mana and 9'1 Allwave are both doable for you, and will be better in bigger surf, but not sure that it will be an overall gain for what you want.
The Allwaves are very well built. The Mana's are not as durable... the paint chips off them very easily, plus I've had delams.
The Allwave is a better deal overall with lower starting prices and the board will not show wear after much use.
The Mana's surf better, in my view, above 9' in length
Below 9' long, I prefer the Allwaves.
BUT.... they are not longboard single fin type ride that you said you are looking for. (maybe an older 10' Mana will do for that)
If you really want a "longboard" feel in a board that will deliver solid surf performance in all waves, consider the 10'6 Nalu/Alana
Tha Alana/Nalu is only 30" wide and will present more of a challenge balance wise, but float is fine for me at 105 Kg plus full 5mm winter wetsuit gear.
I still have my 10'6 Alana and love it in tiny chop or solid OH surf.
It has the "longboard" single fin feel you mentioned. The GS Nalu or Alana are single fin only... and you can pick up an Alana for fairly short money. But buy some "Railsaver Pro" and install before you start hacking the paint off the rails.
One thing that stands out on the Naish Mana's and Nalu's.... the tail rocker allows for very smooth and easy turning with little effort if you stand near the tail, but they surf nicely off the front foot when charging down the line. (Harold Iggy knew what he was doing, these designs are timeless)
For your wife, I think that she will really be happy on a 9'5 Alana (girly version of the mana) or a 9'1 Allwave... way more float than she will need, but small enough to handle off the water and a good solid surfing/paddling board on the water. No need to go really small, she will probably appreciate the extra comfort.
Note:
Naish boards can vary a LOT in weight. I had one 9'5 Mana that weighed about 10.5 K, my next 9'5 mana weighed over 12.7 kg (my 10'6 Alana = 11.6 kg)
Pick them all up.... go with the one that feels right.
Here is a video I did on my 10'6 Alana... not a full review because the stats on the board are well known.
Lots of different wave conditions and camera vantages