Marine Paint vs House Exterior Paint (and more)

[SDHE]

Q1). Why can i not use household exterior or masonry paint, instead of
very expensive so-called 'Marine' paint?
Probably a stupid question, but i do not like spending extra money on
'marine' products if ordinary 'off-the-shelf' products will do (nearly) as
good a job.

Q2). The topsides have taken a beating with U.V, and have started to fade,
is there a reasonably priced product that will (a) Restore the Gelcoat
colouring and (B) protect it from further attack?
Is it more viable to just paint the topsides instead?

Q3). I will be using WBP ply in the saloon for the refit as it is about
1/2 price (or less) of Marine ply. Is it o.k to use this in the cockpit?
Most water will be rain water, but there will be some salt-water spray
when under way at speed / in rough conditions.

Q4). Where do i source insulation material in the U.K? I will be using
thin loft insulation in the roof, with several breather holes to allow any
moisture to escape. I also wish to insulate the hull sides and bilge as
we will be spending several weeks per year sleeping aboard her. What is
the best / economical material to use? Please note that the MAXIMUM
thickness i can use is about 1".

Q5). Any CHEAP ideas for a water heater? I wish to run the hot water off
one engine, and the central heating off the other.

Thanks
Steve H.