Per and Alexander,
I can understand the crashing when using a processing and memory intensive plug-in like Altiverb. Izotope Ozone 5 also requires a lot of memory and processing power, and whenever I've crashed Pyramix by dragging a Library onto the mixer, it's been when either the old mixer state or the new one uses Ozone.
Studying the manual a little further, it appears that I was correct in my assumption that there are actually two different processes available when wishing to save and/or restore mixer states - the Preset (which uses the Library) and the Snapshot (uses the pop-up memu). According to the manual, Presets can be dragged from the open Library tab or window onto the mixer at any time and will update only those parameters common to both mixer states - if a mixer library containing 100 strips is dragged onto a project that is only using 10, only the first 10 strips will be changed and the remaining 90 will be ignored. Presets can also be accessed in the "Organize" tab of the Mixer, and Global Mixer Presets are those that are available when first creating a Project, and Pyramix asks you which mixer Preset you'd like to use. Plug-in parameters are not stored or applied by this method, so overall, it seems to be the less desirable alternative in this instance - being both less comprehensive, and requiring a disruptive mixer rebuild operation.
Snapshots appear to rely more on the Automation routines - although Automation Read/Write states need not be enabled for this to work manually. Because of this, Plug-in parameters, which are "automatable", can be changed along with all the other mixer settings. It is also, I suspect, why applying a different Mixer Snapshot requires less time than a full mixer rebuild, and is able to be applied while in playback or record - it's just the same as though an automation change were encountered at a specific point on the timeline, only in this case, it is manually applied. The snapshot method seems to be less disruptive (or crash-prone) than the Library/Preset method, although demanding plug-ins still slow the transition due to the loading time required (but still, not to the extent of a full mixer rebuild). If only there were a more elegant method of accessing the Snapshots than having to snake through the pop-up and its sub-menus - a macro, a key combination...
Just in passing, regarding IR reverb plug-ins, I have limited experience with Altiverb - finding some years ago that I was unable to incorporate IRs of my own creation/collection, even when they were in the designated file format. That, and the rather high purchase price got me looking at other alternatives, and for years I have been very happy with Christian Knufinke's SIR2 -
http://www.knufinke.de/sir/sir2.php - although it is restricted to 2 channel use (but 0 latency!). For multi-channel IR Reverberation, I'd look at Voxengo's Pristine Space -
http://www.voxengo.com/product/pspace/ - currently on sale for half price. For IR collection, I've been using Voxengo's Deconvolver -
http://www.voxengo.com/product/deconvolver/ - which can create the necessary test/trigger signal and convert the resulting recordings to proper IRs which can be easily imported into either of the other IR Reverb plug-ins without issue. Most likely, both of you are familiar with all of these, but I mention them here for other interested readers.