-slow practice (ultra-slow [no set rhythm, prepare every single movement in left and right hands, focus set on making absolutely no mistakes])
This way of practice, especially with taking away of the rhythm, can be used to solidify exactly what you are playing melodically. It can also help memory (both mental and physical) - IE if you know really know where/how/etc... to play a note - this way of practicing gives you time to remedy that without worrying about your rhythm.
-mental practice (with & without the score; helps also with memorisation)
Mental practice can have many benefits, including understanding your musical interpretation (unaffected by technical issues). Also, as stated, it can help with memorization (more specifically - Visualization). Having the score in front of you and "practicing" without the guitar is more like score study - analyzing and understand the music you are playing.
-sectional practice (micro-phrases [i.e. small fragments], practice separately; phrase-by-phrase; 3 consecutive notes, then 4, then 5, etc.)
Breaking things down into small sections is critical to trouble shooting and fixing problems. Isolating the problem area allows you to deal with that specific problem. Then slowly adding notes before and after allows you to integrate your solution into the musical situation.
-rhythmical permutations - with metronome, with subdivisions; dotted rhythms: 16th-dotted 8th; dotted 8th-16th; 16th-16th-8th; 16th-8th-16th; 8th-16th-16th; quarter(tied)16th-16th-16th-16th
This step often helps me identify finger combinations in fast passages that are awkward or not timed properly in my technique. These super short bursts help to even out rhythmical "swinging" (if the music should not be swung).
-separate the hands
This is great to focus on the mechanics of the individual hands. I'll admit that left hand alone practice most of the time doesn't work well for myself, as there's no sound being produced. Usually what I'll do to work on LH alone, is for example in an arpeggio section take the notes of an arpeggio and play them as block chords. I'm working on the left hand mechanics without the arpeggio getting in my way.
-counting out loud while playing
After you get over the whole "counting while playing" thing - this help not only confirm you are playing your rhythms correctly, but also gives you greater rhythmical control over the piece you are playing.
-singing (singing melody, accompaniment, separate voices; verbalising rhythms from pieces; helps with breathing also)
This helps to isolate and identify the part of the piece you are paying attention to. IE - singing/bass/accompaniment the melody while playing. Also the above listed of "verbalizing the rhythm" and "breathing". With breathing, this allows you to understand what type or how big of a breath you'd actually take if you were singing. That way instead of a mental "I should put a space here to represent a breath" you get the feeling of what the breath actually feels like.
-each note twice (repeat each movement identically – i.e. repeat each RH finger twice per written note; short-long articulation)
I imagine this would be for consistency of sound...? If I were dealing with sound issues this might be a good method to use.
-practice without shifting (stay in one position, but observing all fingerings and string crossings as though playing through the piece)
To me this would need a greater explanation. Based upon it, it seems like you'd play everything the "same" except if you were shifting. Again, as stated above this may be more of an analytical tool for dealing with an issue than an actual "I'm going to sit here and practice" tool. Certainly an interesting thing, but what if the shift creates the problem that you are trying to solve?
-practice with de-tuned guitar (maintain fingering, positions)
This is a pure muscle memory exercise. Making sure your fingers know where they are going.
-separating voices (especially helpful when studying early music)
This is probably playing a polyphonic piece in individual voices: IE - playing one voice at a time thing. This is great to identify where your melodic line is going, especially in a thick texture piece. You can discover some great counter lines buried within a piece this way.
-play through whole piece while purposefully buzzing every note by reducing LH pressure (develops sense of perfect amount of necessary pressure in LH)
This one works on decreasing the amount of tension in your left hand (as stated above)
-practice with cloth/scarf inside the strings at the bridge to mute strings (helps to develop the production of a bigger sound; good for silent/late evening practice)
I don't really agree about the "bigger sound" aspect of this way of practicing. To me, it highlights sloppy RH preparation.
-record yourself, play back (same day or some days later to analyse)
Super useful to make sure you are playing what you think you are playing.
Finding purpose in the reason for choosing a way of practicing something is super important, and there are benefits in each one of the listed methods. I wouldn't practice a piece only in one of these ways, but having them in your "bag-o-tricks" can't hurt. And you may find that one tricky passage may be improved by using one of the "odd ball" ones.