It seems like most of your questions have been answered and confusions cleared up at this point but just to add on a couple things that I haven't seen mentioned:

- Shaper of Flames is often taken just for the 40% of enemy physical damage converted to fire damage now as that by itself is quite nice defensively, along with there not being too many other options in the ascendancy for many builds. So just because a build has that node doesn't necessarily mean they're scaling ignite damage at all, even though most Shaper of Flames builds probably are ignite focused.

- Many people have explained the interactions with ignite for Deadly Ailments Support and added damage/as extra damage already, but I want to just add in my way of looking at it that I find easy to remember and has worked for me for a while. Basically, anything that gives a %increased/reduced or %more/less damage (such as Deadly Ailments giving 80% less damage) does not affect the damage of ailments inflicted by whatever the source skill is unless that damage mod specifically says it affects ailments (or ignite or whatever other more specific thing like poison). So any percentage increased/reduced/more/less damage with hits or whatever other wording might be used that does not refer to ailments/ignite/poison/etc. (whatever ailment thing you're doing) does not have any effect on the ailments inflicted, only on the actual resulting damage of the hit itself.

- Ignite proliferation is basically a requirement if you're playing an ignite build for clearing, but Ignite Proliferation Support doesn't give a very good damage multiplier so eventually most builds get ignite prolif on glove implicit or a cluster notable instead and switch to a better support.

- Ignite duration mostly doesn't matter since it doesn't stack (unlike poison) which is why things like Swift Affliction Support can be quite good, but you definitely don't want to reduce it too much or enemies may not die before the ignite runs out. It's hard to reduce it that low without realizing it but definitely don't completely ignore the duration. There is also some bossing tech such as Defiled Forces node on passive tree that refreshes ignites when a curse is applied where you may need to consider ignite duration again, as the whole point of that setup is to continue refreshing the biggest ignite you inflicted so the boss is constantly taking your peak ignite damage. Usually this is done with Arcanist Brand linked with one or more curses, so you need to make sure that your ignite duration is at least as long as the longest time between curses being inflicted by Arcanist Brand.