Folks buy insurance to mitigate risk and put a cap on financial losses (i.e. life insurance, the fully comp part of motor insurance, extended warranties etc).
You need to asses how likely the bike is to break down out of warranty and what the likely cost is. If you can stomach the likely cost, then dont bother with the extended warranty. Simple.
Personally I, dont bother with extended warranties as I don't think they are worth the money.
With regard to the seat - do you really need it? I find my stock seat fine for short to medium journeys. Anything north of 200 miles I bungee a £10 sheepskin on. Works a treat for me.