Hello,
I would like to say first I think Cornersneaking is a cool mechanic (whether you call it one or not I will be referring to it as such in this post) that rewards game skill and map knowledge by allowing characters specced into Hide and Move Silently to utilize these skills to get an advantage on their opponent mid-combat. Think rogues, hiding away* to pop out a bit later and sneak attack an enemy. I do however think there is very little counterplay to this strategy if you are not a character with spell focus feats in Divination for increased duration true sight. Let's see what the counterplay is:
True Sight scrolls - A True Seeing duration of one round just isn't long enough, and the animation to read a scroll is long and clunky. As I said above, without spell focus feats, it's just a pitiful duration for using during a combat scenario.
True Sight casting - A caster casting true sight has a significantly better chance of catching a sneaker out like this because the casting animation is much more fluid and faster than a scroll. If the caster has spell focus feats in Divination, then they can very easily counter the stealther with the increased True Seeing duration.
High Spot or Listen - Right off the bat, characters that are not elves and do not have 10 or more rogue levels are flattened in this category. Having to stop what you're doing to go detect mode mid combat is not only janky but unreliable as it won't take effect until next round's roll where you will get the full detection skill applied. For reference, characters WITH keen senses or detect mode ON make rolls of [Spot/Listen + d20 +/- situational modifiers] (there are a ton of these and I won't go into them here) vs [Hide/MS + d20 +/- situational modifiers.] Now, characters WITHOUT keen senses and with detect mode OFF make their contested roll at [1/2Spot/Listen + d10 +/- situational modifiers] vs [Hide/MS + d20 +/- situational modifiers]. Their detection is halved, and you will have no chance of ever detecting someone specced into stealth.
For characters that are elves or 10+ rogue, they have a shot of detecting a stealther, but there are many variables to it. Non-wis Spot based characters with skill focus and epic skill focus: spot before clairvoyance or true seeing usually get in the 70's-80's range. A dex-based character with epic skill focus: hide/ms and relevant gear (rings of hiding, other big bonus gear) can hit over 100 in each but with race/feat choice can push it even higher than that. Characters speccing into Listen have a MUCH better chance because of the Amplify spell, which adds a whopping +20 listen for 1 minute per level! This means 7 minutes per use of an Amplify wand or potion or scroll. This is a gigantic bonus, and Listen-specced characters using Amplify consumables will usually blow away a stealther in the detection game.
Options 2 and 3 have their merits, but Option 1, scrolls, falls short. This True Seeing effect is also the only way many, if not most, characters can ever hope of being able to detect a stealth-based character. For discussion purposes I think a True Seeing base duration increase to 2 or 3 rounds would help alleviate this issue and give Cornersneaking sufficient counterplay to characters utilizing scrolls, because it can feel overbearing if you are someone stuck using a scroll due to the fact you are completely at the mercy of the cornersneaker at all times of the engagement.
As an aside, I think amplify is a bit too strong. +20 to a skill for turns/level is absolutely gargantuan, and anyone can use this spell for a duration of 7 minutes with each use. Halving it to +10 or making it rounds per level would make it much more fair, in my opinion.
*Whether it is nonsensical or not logically for someone to vanish this way, frankly I don't care, because this is just a video game.