Rory,
I think you should stay out of the water for a bit while you give this a try. Like I said it's a longer term thing, so the effect is negligible if you subject your sinus to pressure every day or so after you take the Guifenesin.
So give it a try starting now and for at least a week don't go in the water, pool or ocean. Do dry static and land based exercise for a week while you give your sinuses a chance to calm down. Cut out all dairy, etc. also, anything that contribute to this. After a week, try submerging at the pool slowly. If there is any difficulty stop immediately and stay dry again for another week.
I know it's very hard to contemplate, but I had to do the same thing. When I first got my sinus squeeze I thought if I descend slowly enough I could 'push' through it. Well, it was literally squeeking by and I thought I was okay, but on the second day blood started coming out and I was smelling blood with every breath. It was pretty bad. I went on Guifenesin and Afrin (for a few days) and stayed out of the water the whole time, which sucked hard because Cayman viz was 30-40m+. But after 8 days it suddenly went away and I could dive again. Since I equalize hands free I use a lot less pressure so I'm pretty sensitive to something like this.
I had a friend in school who had a pin put in his little finger when he broke it. He loved playing guitar. His doctor said to not play for a few months. He played anyway a week after the surgery because it wasn't really painful to him. Well, he tore it up within days and now he couldn't play guitar anymore because he permanently lost his little finger dexterity.
Point is it may not seem like much, but treat this injury very very gently. There are small delicate bones and passages up there. Don't want to make the damage permanent.
BTW, I'll probably be there saturday or sunday as my work schedule doesn't allow me much leeway.
Best,
Peter