"Boomtube - Is excessive pressure the ONLY thing that can cause sticky extraction?"
IF I understand your problem, yes. The "sticky" only occurs after firing, your new cartridges chamber and extract easily if unfired, right? If so, everything that happened in firing is pressure related, including spitting a bullet out.
When the pressure is too high, the case AND the CHAMBER are both overly expanded. The over expanded brass case springs back less than the steel chamber, so the case gets lightly gripped by the contracting chamber and you get sticky extraction. That takes a LOT of chamber pressure! In my opinion, your action is doing a GOOD job holding together now but don't keep it up or it may give out on you. Violently!
A too smooth chamber and/or residual case lube will certainly increase thrust against the bolt and that may eventually damage the bolt lugs or seats. But, I can see no way that bolt thrust can, of itself, cause sticky case extraction from a mauser type front-locking action.
Some people have stated (on the net) that velocity will, or can, increase suddenly and significantly when pressures reach dangerous levels. THAT'S NOT SO! High chamber pressures usually skyrocket unpredictabily with very little increase, if any, in velocity. Especially with heavy for caliber bullets!
Don't try to understand chamber pressure by any chronograph results, your cases are telling you all you really need to know at this point!