Yes. And for all intents and purposes, chained attacks are true blockstrings, which means there’s no gap between them. There’s the odd exception in certain games, but no need to worry about that. The other thing is that not all chainable attacks can be cancelled into each other, even if they are marked with the chain property. You should test this in training mode with the character you play.
I don’t know much about Chun Li in SFV, but this should be easy to test in training mode. Just record the dummy doing cr.lk cr.lp against you as fast as possible and see what happens if you let go of the stick after blocking the cr.lk. If you stay blocking then it’s a true blockstring and in all probability a chained attack.
Yes, the frame data indicates there’s a 4f gap between the cr.lp and cr.mk on block.
There’s no such thing as a “true” frame trap. Different frame traps can be used to “trap” different buttons, in the same way that you use different traps to catch different animals. What they probably mean is that the opponent can beat this with a 3f light attack, which they can’t do against cr.lp st.mp. In SFV, normal moves of higher strength beat weaker moves even if they connect on the same frame, so even there’s enough room between cr.lp and st.mp for the opponent to slip in a 3f light attack, it will always lose to the st.mp.
But, because players in general know that they shouldn’t press buttons after cr.lp, it makes cr.lp cr.mk also usable, especially because it hits low, and is hit-confirmable (don’t worry if you aren’t able to do this yet).
Furthermore, against characters who don’t have 3f light attacks, cr.lp cr.mk is generally fine, although it could trade with certain medium normals (which isn’t a completely horrible thing).
Of course, the great thing about the cr.lp, st.mp sequence is that because you’re now at +3f (after the st.mp), you can follow-up with cr.mk for another frame trap. This is of course predictable so you need to mix it up.
Also note that invincible specials with enough speed and range will beat frame traps.