Nobody has ever tested my draft. I would say it’s not poor draft, but if anything I’ve worried about overdraft. Actually the backpuffing seems better if I engage a key damper. But then again, I like blew up that one time.
Liner is closed off at clean out.
This is a bit of a summary, strategy, and my own interpretations to make the case a bit clear, as it is not completely clear to me what are single instances of something happening ("that one time") and what is a consistent feature.
You have a consistent issue with warping pieces and backpuffing. I do not know whether the circumstances have systematically been addressed.
To nail down the cause of a complex system that misbehaves, one needs to be very systematic. Eliminate causes one at a time, honing in on the issue.
This does not always lead to a clear answer, but in my experience it is the course most likely to lead to an answer.
Hence, first look at the fuel (experience tells us many problems are due to fuel). Please measure the moisture content.
If that is not an issue (we don't know yet), experience tells us user behavior or flue system are more often the issue than the stove itself (if from a known, trusted brand).
So second is to look at draft, if at all possible. You are working on that with the kind help of someone here. Good. (Do see if you can keep in touch with him about how precisely to do this - we want the data to be as good as possible.)
Though 25 ft after an 1.5 ft rise, some horizontal piece, and two 90 degree elbows should not be overdrafting imo. Could be a bit on the high side of the recommended range, but the elbows and horizontal section, as well as the bit short first vertical rise, will kill some draft of the "too much. 25 ft".
However, counter the above "not likely a problem", is the observation that you consistently end up with warped pieces.
With user behavior I mean in this case the "don't close the air too fast, but in small steps spaced out in time". Closing too fast could be instigated by too high draft too (it roars, one gets scared and closes the air - too rigorous). The result of that is that a lot of fuel is outgassing (it's hot enough to produce smoke/gases), and after closing the air there are not enough flames to consume most of that, resulting in a build-up of gaseous fuel that if the mix is right occasionally explodes. Given that the stove is not a tight container, it's not like a bomb, but it'll spew smoke out of the air inlet(s), anywhere else with low pressure tolerances, and pushes it up the flue. That is backpuffing.
As bholler said, backpuffing is more common with poor draft (because the other way to describe it is that there is not enough oxygen coming in to burn the gases - because it's not being sucked in due to low draft in a poor draft case). Therefore measuring draft is before this.
Regarding the observation that the OAK made the stove run hotter, that suggests to me you may have "high" draft and a tight home. The home dampens the air flow, but with the OAK that damping is not present, and thus it runs hotter. (An open window would do the same.)
In any case, I suggest you do measure the moisture on a (room temperature) piece that you freshly split. It's important to know for the issue you have, but also for burning cleanly (and thus safely) - and for the heat output (you don't want to be using the heat to boil unnecessary water out of your system). It's good to do this as soon as possible.
Summing up the observations:
The OAK=hotter, warped metal suggest high draft to me. The backpuffing could then be fuel wetness and/or too quick closing of the air.
I'm sure there are others who may disagree and who may be more knowledgeable, but this is how I see it.