1 & 2) Good point. This class draws on some especially obscure sourcebooks too, so it's useful to have a key to the acronyms. I've added a spoilered section at the top of the class listing sources and acronyms.
3) Invisible spell does exactly what you think it does. However, it's a +0 metamagic feat, so the only thing I'm letting the player get out of is the prerequisite, which is merely to have one other metamagic feat. Further, it's always on, and so less flexible, and the PC doesn't actually gain the feat, merely its benefits, so she can't, for example, use it on spells gained from another class or use it to qualify for other feats requiring one other metamagic feat.
Moreover, the feat is less effective on the void disciple than on other casters, because most of a void disciple's spells don't have a visible effect anyhow. Its chief impact is to hide the rays created by the various ray spells. In that regard, it's mostly a cosmetic change intended to help me reflavor a lot of the spells whose effects are appropriate for void magic, but whose flavor or appearance didn't mesh.
However, I'm open to the idea that there may be some loopholes for abuse I haven't considered. Are there any spells on the spell list that you think are made grossly more powerful by having invisible effects?
4) Sense Void lets you perceive as if you were standing there. However "there" is wherever want it to be within range, provided you know where "there" is, and you can reposition your "self" as often as you want to while sensing. Sense Void is the flagship power of the Void Disciple and the reason that it's an extremely powerful class despite having a very limited spell list. I've actually nerfed it pretty hard by increasing the skill check DCs dramatically (on the original class, the highest DC was 25). However, I did allow the use of some additional spells at higher levels, to tie the void disciple spell list into the ability more closely. Do you think it's gone too far? Should I place some limits on sensing or establish some game elements like warding spells that will prevent sensing?
Perhaps I could make each progressive level of sensation add +10 to the skill check DC, so that a DC 45 check will get you physical senses out to 1000 miles, or magic senses to 100 miles, or empathic senses to 10 miles, or mental senses to 1 mile? This would mean that mind-reading the BBEG from 101 - 1,000 miles away is a DC 75 check, which, while not impossible for a high-level void disciple, while require the expenditure of more resources in the form of void points, spells, or skill-boosting items.
Edit: And more critiques, please! That's why I've posted it here. I've made quite a few classes over the past few months, and every single one of them has been greatly improved through feedback from the community. Not one was perfect when first posted, so I'm expecting and hoping for feedback on how to improve the void disciple.