Xylene forms an azeotrope with water [the mixture boils lower than water], a few small traces are very easy to cook off of a solution. I use a coffee maker hotplate, as it heats water to nearly the boiling point of water-xylene without ever reaching the boiling point of water. It works so well I'm not afraid to drink the resulting water, in small single-dose amounts. Thats how I avoid messing with DMT goo or bothering to get DMT fumarate crystals.

I hate undershooting the pH during mescaline saltings, even with HCl since it leads to a less pure product as you noted [that yellow leads to orange mescaline and its a sticky black goo in its pure state] and I dont like having to add base to a solution that should just be mescaline and water. Theres one trick I use to minimize the risk.
Mescaline is somewhat soluble in water, you'll experience this when salting because you'll add a little acid and the water will still be like pH 8.5 due to a portion of mescaline freebase being dissolved in the water. While one can pull all the mescaline into water in one salting, with luck hitting pH 7.0, what I more often do is salt some of the mescaline into one portion of water, test the pH and set it aside, then do the same with another portion of water and add it to the first. These should be pH 7.5-9. Then if you undershoot on the last salting 1) it'll only effect a fraction of your mescaline and a smaller volume of water 2) you can slowly add it to the previous combined portions while monitoring pH, that free mescaline will help neutralize the excess acid 3) if you undershot by very little the final mix will still be basic and you can very carefully add the last needed quantity of acid without xylene being in the way, complicating things.
The pulls taken off when its still basic are quite clean, that yellow gunk comes over a little at neutral and a lot once you reach and go below pH 6. Whatever it is, its a weaker base than mescaline.
Its a good idea to have some very weak acid for the final stage of salting. I use 1M HCl for most of my salting, but I switch to 0.1M HCl at the very end [the equivalents with sulfuric acid would be 0.5M and 0.05M. 10% sulfuric acid is 1.02M, so not bad for most of the salting if your careful but some weak stuff would help at the end.]
The yellow/orange gunk can be removed by recrystallization or anhydrous acetone washes.