Question 1: go for a disc. Did half challenge Almere twice, with disc, no problem (71-74 kilo here, bit shorter at 178). Your weight is on the back wheel anyway, and it cannot turn (like your fork), so no problem. Front wheel depends on wind and you abilities, but is more important in the light of handling wind. For me, till 3bft I ride a 90mm front, 4-5 I'll switch to a lower (20 or 30mm, since I don't have a 40-50mm wheel that fits my TT)
Question 2: yes, an aero helmet will be faster. That said, there are many options. Take the following into the equation: temperature, weight, with/without visor, aeroness (not perse in this order). What I mean: fast helmets can be hot / heavy (like the Giro Aerohead). Cooler helmets can be slower (normal aero road helmets, or something like the casco speedairio, which is my choice for the full distance). In general the aerohead is a fast helmet (albeit quite hot). A specialized evade would be a fastish road helmet.
Question 3: eat more. Trial and error on you longer rides. Think abount mixing gels and solid food if you can't get enough gels (or whatever you're using now) in. I prefer mixing some gels in a botle, some gels in a bentobox and some solid food (bars, broken in 2, so no hasle on the bike)
Question 4: for me, it doesn't matter that much. But I'm a slow swimmer (1:07 I hope). Guess you burn more when you're pushing it, might eat something extra upon exiting the water and/or have a extra gel before swim start (not directly before the start, maybe 20 mins out, with some water/sportsdrink?)

5: see you there!

Thanks for the advice! The nutrition is a bit of thing with me. I cannot seem to figure it out.