Before you do any further overclocking, I'd recommend looking over your UD3 BIOS settings and getting as familiar with them as possible. Dealing with CPU speed is fairly easy - just mess with vcore and maybe LLC settings. NB stuff can be a wee bit more complicated.
Question 1: Depends on your thermals. Increasing NB speed/voltage doesn't do much to increase heat flux/temperatures, but NB stability decays pretty badly as die temperature increases (in my experience). With your temps, I would say 2400-2700 mhz NB is achievable. It depends on how much CPU-NB and NB voltage you want to give it. C3 chips can hit higher NB speeds than C2 chips.
Question 2: I would recommend a simple 5-10 minute Prime95 Blend or Linpack (using all memory) session every time you raise nb speed by whatever increment you decide to use. Keep doing this as you raise NB speed until you get problems booting, then back it off one step and do maybe 4-8 hours of Memtest 86+ tests 1-5/1-6 followed by 12-24 hours of Prime95 blend/Linpack/whatever.
Bear in mind that instability from NB problems will manifest itself differently than CPU instability. Sure, it can produce failure to post, though it can produce other annoyances, such as the claim that your BIOS is corrupt (this is a real PITA).
Question 3: not familiar with the UD3 BIOS. I would recommend starting with the
manual to figure it out for yourself.
Question 4: HT overclocking is not recommended (note I'm referring to HT, not HTT). You will not see any increase in performance from increasing the HT.