Scholar techniques

[Tranquil]

If you use an instant skill (aetherflow, sprint, lustrate, swiftcast, cleric stance) while you are casting a spell the instant skill will activate immediately after your cast is finished so you don't need to spam press the button.
[...] The WD will have the rouse effect provided you use it at 50% cast.

This is true and pretty useful. However, as far as the WD goes, I don't know if you are using 50% as an arbitrary easy reference-point or not, but it's good enough if the buff is applied at literally the last possible instant for the game to register it being up there, even if the cast is nearly going out. But mayhaps 50% is a good reference point for this, so yes.

The deactivation of cleric stance does not have the short delay that many instant cast skills do. You can use this (and the previous find) to create some interesting combos:

(in cleric stance): ruin (cleric stance) -> lustrate vs. ruin -> cleric stance -> lustrate
result: ruin -> cleric stance OFF & lustrate together
this is slightly better than deactivating cleric stance after ruin goes out and then using lustrate because you don't have to press two buttons in quick succession which is not as optimal.

What you say is true, but it is mostly useful intel for those people with a case of "chubby fingers" where they double-tap Cleric Stance, de-activating and then re-activating it, instead of having a feel for it going off the instant your cast has ended.

Spell casts will still complete even if you are interrupted at approximately 80%-90% cast. This allows you to move slightly before you complete your cast.

This is incredibly elementary knowledge. (Not to diss you or anything). One just needs to be careful they don't do this too haphazardly, because it is also possible to have the cast visibly go out with its animation finishing, but no effect going out. It's all in the finetuning though, and this technique should indeed be used.

Fey Illumination increases your healing output, not healing received.

This is true. You can test it easily by casting Fey Illumination solo, then casting heals on random people in the vicinity - you will notice you're healing 20% more. Good information to know if thinking of Eos placement for optimal ranges - trivializes that part a bit, somewhat. Still, good points and thread.

As for myself, I can't really think of any "techniques" right off the bat, but I'll add a soloing-questing tip, mainly for dailies:

Use your fairy as your tank, if you are doing some tedious dailies wherein you get monster spawns/random mob aggro, when you have to interact with things.

Easily achieved by having your fairy manually cast a heal on you (use a manual Embrace keybind) and without you doing anything - this will then make your fairy rip off hate off you, gravitating the enemies towards her. Open-world trash is weak enough that you can even have 5+ of them gang up on your fairy and she will be able to survive Embracing herself. One should preferrably place her a bit further away, as some mobs have AOE-cones that will then interrupt your interaction again.

This saves you a lot of time in the long run, as opposed to disposing of the mobs. Will edit as I remember things worthwhile a post.

Edit:

Another quickie for those potential fairy-swaps:

Using a fairy's cooldowns and then fairy-swapping (Eos←→Selene) will make it so that the cooldown duration is adjusted to be the same as the previous fairy's skill used.

This might seem like common sense to some and less so for the others. However, what this basically means is that, in your normal pet hotbar-layout, the three buttons to the right of Embrace both are activated from the respective fairy's hotbar with the activation of either. So basically, the "pairs" are:

Whispering Dawn & Silent Dusk
Fey Covenant & Fey Glow
Fey Illumination & Fey Light

If you have Eos out and use Whispering Dawn (60s CD), then instantly swap to Selene, it will then have Silent Dusk on a 50s+ CD, even if Silent Dusk is normally on a 40s CD.
If you have Eos out and use Fey Illumination (120s CD), then instantly swap to Selene, you will find out that your Fey Light will have 110s+ left until it is back up, even if it is normally on a 60s CD.
If you have Selene out and use Fey Glow (60s CD), then instantly swap to Eos, you will find that your Fey Covenant will only have 50s+ left until it is usable again (being on a 120s CD normally).

Point being - be careful with how you use your fairy-CDs if you plan on fairy-swapping. Not many people fairy-swap mid-combat, but for those of you that do or plan on doing - keep this in mind. It's not really that important, because if anything, people usually swap in the order of Selene→Eos, hence the 120s CD Eos-cooldowns will be available after a shorter time of inactivity due to Selene's CDs being shorter. In reverse, though, it can have more impact. Just closest to what I could immediately think of for this thread.