We've had 3.1/3.4/3.5/3.6m sibs and used 4/8/9.8/15hp motors with loads from one up to two adults with two early teens and a dog plus kit for a day out.
As others have said and you have found with 2 16 stone adults you are on the frustrating edge of plane/no plane. In this circumstance every small factor really adds up such as wind and wave state... but most of all every extra pound you carry makes a huge difference.
Unfortunately the Honwave is quite heavy for its size assuming you have the alloy floor... it's about 25% heavier than a similar Zodiac. Your outboard is about 15lbs heavier than the lightest in class and if your transom wheels are the flip up type that stay on they add a bit.
Doel fins have never really worked for me as they introduce drag with the lift they give so 50-50 benefit/no benefit.
I assume you have the boat right up to pressure and have checked on the gauge after it has been on the water a few minutes? Nothing (apart from your mate) you are carrying that can be left behind? How much fuel do you carry and where is the tank? Where do you both sit and do you try moving about as the planning point is reached to get a balance where it will try and jump away?
If none of the above leads you to an answer I'd get a finer pitch prop. I've swapped props about on similar outfits and the change is very noticeable... even a new standard prop will improve on a very ragged old one. Agree a 4 blade will not help... probably be worse. Also bear in mind there can be a slight difference in blade profile between an original and aftermarket prop which with the knife edge you are on and add to your problems. I would stick to an OE one if you can.
Frankc knows his stuff but I would have thought in the UK your 9.9 might have been a 9.5" pitch as standard.
In any case I would give Steel Developments a mail/call... Home page
They know so much about props and in my experience give honest advice. They would want about £70 for a new prop or £40 if they have a recon one in the pitch you need... their recon is as good as new. They would recon your own for about £35 and I wonder if they could reset the pitch to whatever you decide is needed.
I found this Evinrude prop guide that is of general interest and on page 17 refers to the prop choices for your outboard...