Is Toxikon a dps gain?
Toxikon is DPS-neutral on one target at level 90 and a DPS gain on 2+ targets if you already have a Toxikon charge. Using Eukrasia Diagnosis and then Toxikon is a loss on multiple targets compared to Dyskrasia.
What is the purpose of Pepsis, will I be using it a lot?
No, Pepsis is a utility button but not something you will be using as part of your healing rotation. It’s a good way to clear shields if an incoming AoE needs it and your shields may fall off right before it happens.
Can I double weave after my DoT?
No, Eukrasia has a 1s recast, while the following spell has 1.5s recast time. You can only single weave after the second spell.
Do shields stack?
The image below shows conflicting shields and which shields win out, anything not listed will stack. (Thanks to Meru for the image.)
How do I fit Phlegma into 2-minute buffs?
This spreadsheet by Aeya should give you an idea on how your 45s cooldown works with buff windows. Be aware fitting in your second Phlegma in 2-minute buff windows is really tight.