In the interest of harm reduction a .22 micron filter is necessary for your safety.
In the interest of reality filtering through a wadded up q-tip after heating to just below boiling is more than adequate. I've been injecting dissociatives intramuscularly for 15 years with nary a problem with this approach. (as well as a good several years of injecting less savory, dirtier shit with no problems either

) Not really rec'd, but it seems to work ok for most if you are unable to acquire wheel filters.
And the drug testing question (not allowed! ).......If they do test for ketamine (and that is a real big if....very unlikely) they will be testing for the gluconurate. This will be an enzyme test, followed by a GC/MS. And it is a possibility the EMIT
could show the MXE as K, but impossible for the GC/MS, and this last one is what is required for a positive. You should be fine.
And the "respiratory depression"....Ketamine can both depress and stimulate breathing. But at normal doses there is no considerable depression. But there can be an "apnea" style situation, that can feel and sound like respiratory depression, and can depress O2 levels, but not to a significant degree, unless combined with other depressants. I've experienced this with ketamine and MXE, and it feels or sounds more dramatic than it is. It really sounds like some labored breathing, but I have a feeling your O2 sat is only down a few points if at all.
The MOR activity question again....I dunno. It is all speculation still. It sure doesn't seem like MXE has any appreciable effect on the MOR, for the reasons I've stated earlier in the thread. But from a SAR perspective it sure could. Look at tramadol. and then look at methoxetamine, very similar in some ways, especially the supposed MOR pharmacophore for tramadol. that 3-MeO group. This is demethylated in tramadol, creating a stronger agonist. (and happens in codeine to morphine too) This could be happening with MXE, producing 3-HO-MXE. This could be a more potent agonist, or an antagonist, or neither. We don't know But me thinks it doesn't activate the mu to any noticeable degree.
Cheers