I once bought a Beuchat wetsuit that the edges of the pants were not stitched (factory default). When I saw that I asked the seller to stitch it since he had a stitching machine, he replied that it is not necessary but he will do it anyway. I trust the guy and I believe wouldn't have had a delamination problem since then, after 2 years of extensive usage. If you think about it, the whole surface of fabric is glued underneath so it will hold even the edges.
Now, about the cutting part of your post... as other people pointed out you will lose heat faster, you also have to consider that this part seals the water out and when cut the difference will be major, also consider that the 3.5mm thickness will fall over usage. When fishing/diving for 3+ hours the colder water will eventually penetrate, plus when doing deep dives you will need to relax a lot which means less movement therefore less heat produced from your body and it is at those deep dives that you need to be worm or your body will have to spend more energy by burning valuable oxygen.
Long story short it is better to feel overheated than cold, when overheated just suck some water inside your suit, if you are overwhelmed the bottom part of your jacket (at the height of your waist left and right) will give you enough inflow, you also have the sleeves and hood option. There are options to drop your temperature but not the way around, I use a 5mm wetsuit even at 26 celsius (not recommended but still... bearable). On the other hand I had dive yesterday at 21 celsius and at the beginning I was ok but after 1.5 hour I felt the cold penetrating.