here is a simplistic response from my personal experiences with gas kilns since 1967. Didn't fire kilns in '66, just made things.
1. Downdraft & Updraft Kilns
I prefer down drafts for easier control of getting even reduction in the kilns. I learned to fire on home made downdrafts while in College in the 60s. I built several down drafts in Grad school for soda and salt firings and and oil burning kiln fired with modified burners. Also fired 2 Alpine updrafts in grad school..and found they needed a different approach to stacking to help even the bottom with the top. All this comes from repeated firings regularly and through different weather conditions. In 2011-12 I fired a huge Olsen updraft with 14 burners entering from the bottom. With the guide of a 20 year kiln log from the previous professor, it fired perfectly even. I'd say updrafts with burner ports coming through the floor are preferable . I still prefer downdrafts. I designed and built all the kilns I used during my 25 years of teaching in Montana. -2 burner downdrafts: a catenary Arch and a Sprung Arch. .First 5 years were forced air. Second 20 years were venturi burners on a large car kiln and a sprung arch.When I taught in Hawaii, they had a program of hundreds of students and we were firing many dozen or so updrafts during the week.- team firing.The success is dependent on stacking and balancing the burner output with the dampers.
2. Forced Air Vs. Natural Draft Kilns
In higher altitudes I think forced air is a benefit. you need a higher stack (chimney) if you are using a natural draft. Mandatory.
3. Flame Paths Inside Kilns-
flame paths are dependent on the stacking. Cross drafts, updrafts, downdrafts all follow the maze through the ware to the flu. In the kilns I fired for 20 years, downdrafts with ventures (and all kilns are unique) I stacked tighter along the back wall, 9" on the bottom course and tighter in the middle and top front. Think maze and flow path when stacking.
4. BTU Distribution Inside Kilns
The BTU distribution? Not sure what you mean unless you are referring to hot spot. BTU needs are measured by the type of insulation the kiln has. BTU measurement is the burner output.
For example, a kiln with quality insulation brick requires (ballpark) 14,000 BTUs per cubic ft. at max capacity. A hard brick kiln requires 30,000 BTU output because the hard bricks have mass density and need to be heated up more.
Just my experience. I fired regularly in east coast Philadelphia, Midwest Carbondale, Ill. and Billings, Montana (3500 ft ) , Honolulu, Banff (6,000 ft) up state NY. Also the differences of propane and natural, diesel, can determine firing behavior. Diesel burns inefficiently when it enters a cold kiln. Need to avoid carbon clinker buildup. Wood is another case all together.
A book you might want to read is "The Art of Firing " by Nils Lou.
At NCECA in the past, many conversations occur regarding your many questions. I think the conference will be help in 2018 in Pittsburgh. You should try to attend.