In other words, my idea won't work, but I won't understand why until I learn to mod? Once again, ambition is struck down by lack of knowledge.
Here...yes a little bit unfortunately. Im also very new to DF modding, but a step I made sure I took was to haunt this forum for a month flat so I knew what others were doing, how, and where all the big pitfalls were.
The things you are describing are awesome ideas. But to make that sort of thing work you would not be writing a DF mod, but a completely independant game. The things we can actually affect as modders are a tiny sub-set of DF's mechanics. The rest is all hidden, and probably we wouldn't understand much of it anyway, thats what happens when you have a 1-man team coding a project for a decade. By the end he's the only guy who gets it....and often then only kinda. We cant add new features, we use existing ones to 'fake' it to the point where a player with no modding experience thinks its all legit.
In this case there are a couple of issues with the flamethrower. We cant mod seige engines at all, we cant really create custom liquids. Animal and plant extract yes, but pumps only work on water and magma, we cant mod pumps or either of those 2 fluids. We cant really do an ignition interaction, most things that go boom in mods are caused by temperature change (you can make an arrow 'explode' if you have it boil at body temperature and it sticks in. In practice it doesnt explode, just boils off harmlessly, the trick is adding a nasty syndrome to cause the target to die in such a way people think it was the arrow going boom).
My advice would be to trawl the modding questions thread, look through the release threads for some of the major mods. The modders working on these often discuss the troubles and roadblocks they hit in the threads, and the modders question thread tends to be full of useful bits n pieces like this.
Ultimately you are often going to hear 'NO, never, impossible, cant be done' and variations on this. Its not that we are mean, its just that theres a lot that we cant do. Some of the most impressive stuff in major mods is done by some extremely ingenious and surprising behind the scenes ways.
The classic (and for me very ingenious) hack-about is the creature transformations everyone is playing about with. Generally a custom workshop, special reactions. Dwarf grabs ingrediants, heads to workshop. *poof* hes a mage, or a steel golem, or a necromancer, you could even turn him into a cat.
Seems all very simple from the outside, until you realise that Toady's DF does not in anyway support this kinda thing in a logical way. Most of these latest shenanigans revolve around reactions producing stones which immediate boil, are inhaled by the working dwarf and a syndrome actually does all the cool stuff. Workshop and reaction are just there to create said rock and make you pay appropriately for it.
So...dont give up, lurk around this forum a bit. Continue to post your ideas but dont worry too much about the fine detail yet. Chances are DF doesnt work the way you thought it did and someone might suggest a way this can actually be done.