The healing system is pretty lenient as you note, on the highest difficulty you can still carry nearly 7 lifebars worth of healing with you. On Normal it can get as wild as 14 lifebars IIRC.
> breaking guard
Each weapon has a unique stat relating to this, some weapons break guard more easily like the Dabi while others less so. This also goes reverse by the way, with some of your weapons having an easier time breaking their own guard when blocking with them. Note that if your guard is broken you can just let go of the guard button and press it again, this is called a guard-reset.
Personally I find it best to just keep on the pressure or to throw them.
> charged Y
The technique is called an On Landing Ultimate Technique or OLUT for short and it is a key component for high level NG and NGII gameplay. Managing essence is a big part of the gameplay. Do you let it float (by holding block), do you absorb it (by doing nothing) or do you use it as fuel for a stronger attack (OLUT)? Lots of microdecisions.
Note that Essence's value is partially decided this way too. A regular absorb gives you 100% of its value, while an UT absorb only 15% (iirc). While a foe killed by an UT gives an essence orb that is 400% of the value (iirc, don't quote me on these values). Note that a foe killed by an UT can ONLY drop yellow essence, so if you're hurting for a blue-orb to heal, keep that in mind.
> multiple enemies
Divide and conquer for the most part, enemies aren't that spongy. Note Ryu's mobility and the generally large arenas. Dodge around, put distance between yourself and the groups and single out a foe. Wall-attacks are very overpowered in this game but be wary as they can also breed bad habbits (panic jumping).
> blue after image
It is just fancy. Most of those moves have i.frames, but it isn't consistent at all and some moves that don't have it (XXY with the lunar for instance) do have i.frames.
As noted, the ones Tai pointed out. But there are a ton of i.frames in every combo during certain parts of the animation but that's all just a safety net to make those combos more viable in the thick of it. Experimentation is key here. Sometimes it is literally a frame or two.
The best movement is dodge > jump, at least starting out. Later-on "continuous jumping" is key, which means pressing Direction+A on landing, this lets Ryu jump extremely fast: https://youtu.be/tB9NfMYoebQ?t=386
(mechanics are generally carried over in the games, this was the only footage I could find quickly)
If you have any more questions, hit us up!