Eric, I don't particularly like the glissandos either
Regarding the use of grace notes, or not using grace notes, there can be several reasons for the inconsistencies. For example, the grace notes can be part of the original composition, but the slide marks added by the editor as they are specific to the used fingering. It could be said that grace notes are more 'mandatory', and the slide marks fall in the domain of interpretation.
There's also the difference that the grace notes generally fall on the beat, when they precede the main note. When the grace note is added after the main note, as in bar 14, it means to slide and then to momentarily keep the note ringing before actually playing the next note. In my opinion, for example the 2nd beat of bar 14 could be substituted with a dotted quaver chord that slides into a semiquaver chord. If the grace note was replaced by just a slide, it would mean to start the slide slightly later and play the next note immediately after arriving at correct fret.
Bars 6-7: Adding a grace note 'e' to the end of bar 6 in my opinion would make the slide earlier so that the 'e' would have a chance to ring momentarily before actually plucking it along with the bass note 'a'. Adding a grace note 'd' to the beginning of bar 7 (effectively moving the slide to bar 7 alone) would mean that 'd' was to be played (again) on beat 1 of bar 7 at the same time with bass note 'a', and to slide to 'e' only after that.
Bar 19: Here is the same problem. The use of a grace note after the first crotchet would call for glissando that ends up at 'c' slightly before the 2nd beat. If the grace note was placed before the 2nd beat, it would mean to pluck 'e' then slide down to 'c'.
In essence, the difference is, the grace note always has a duration of fixed pitch (i.e. non-sliding duration) which is taken either of the previous note (when the grace note is connected to the note preceding it), or the next note (when the grace note is connected to the next note). When just a slide is marked, there is no duration to be spent at the destination before playing the note, but the note should be plucked immediately on arrival to the destination note, and the arrival should happen exactly on the beat. As discussed before, it's up to the performer whether to slide audibly or not when no grace note is marked. The notation used by professor Delcamp does not seem to differentiate, and I think that's a good thing, as it will give more freedom to the performer based on individual taste.
There also seems to be a convention that a grace note connected to the last note of a bar is moved to the beginning of the next bar. I believe that is a restriction of the software used for the sheet music. Generally in music the grace note always precedes the main note, so I think the only way to accomplish a grace note following the main note, is to attach the grace note to the next note, and add a slide marker between the grace note and the previous main note, which is why the grace notes that should be the last notes in bar move to first notes of the next bar. That's a clever workaround which works as long as the piece doesn't end up with a grace note
I've probably said all this before, and you've disagreed with parts of it, and it's very possible that it will all happen again the next year