just to confirm, you purchased the EC stencil mate ?
which oven are you talking about ? I can't believe there's such a big difference of temp with a small offset with 3 heating bars, on the EC (plus moving air with the fan).
I reflowed a backplane board with a 90 pin BGA connector, the board was like 30 x 20, no issue, really. I'm also placing several panels in the tray, up to 4 currently and this occupies 80 of the tray space : reflow result is totally even with beautiful solder joints.
Also, the profile is the thing. I currently have a 235° reflow temp in the profile to guaranty I get the right temp *on the PCB*. I could attach the additional thermocouple for a better precision but I found that increasing a bit the peak temp just works fine and I had no component failure : the temp on the board itself will always be less than the air. When I watch the reflow phase, I see the solder changing at about 1/3rd of the reflow duration, one heavy / big part reflows later due to temp inertia, in the middle of the phase but overall the duration is just enough.
Reflow duration is 1 minute in my case, but the actual dwell is a bit less than that as the temp must first reach the peak temp, so it's probably more like 45-50 sec, still < 75 seconds indicated by the datasheet.