Why would a 1.74 index lens be more prone to chip and be less resilient than 1.67? My optician counselled me

I don't recommend a 1.74 index with a metal grooved frame, like the one you want from ic! Berlin, because the edge will be less resilient and be more susceptible to chipping.

Please explain like I'm 5 years old. I don't know physics or opticianry. Why's 1.74 more brittle? Why does "chipping along the groove" mean? What does "thinning" mean?

Think of lenses as having 4 levels of thinning; Basic 1.49, 1.59/1.60, 1.66/7, and 1.74.

Why would 1.74 be too fragile? Why a lot of stress on the lenses for metal groove frames like those?

1.74 will most likely chip and break especially along the edges of the groove.

My optometrist, different person, diagnosed me with nearsightedness. His prescription shows

OD –6.00 (Sphere), –1 (Cylinder).
OS –7.00, –1.

 A: This appears to be a materials issue rather than a pure physics issue, so I will be speculating here.
There are two ways to make a more powerful lens: grind it to a greater curvature or use a material with a higher index of refraction.
If you use a greater curvature, then the thick part of the lens must be thicker.  The lens will have more weight, and the support of the lens will be under stress.  So there is benefit to using higher index materials.
(Now for the non-physics speculation)...  The refractive index isn't a free choice, it's determined by the materials and manufacturing process.  So probably other parameters (such as hardness, structural strength, cracking, etc.) may be affected.  This isn't explicit in the discussions you linked, but I read it as the materials used for the highest refractive index had drawbacks in how they behaved when mounted in frames.
Your prescription shows the strength of your lens in diopters.  For lower-strength prescriptions, even low-index materials are acceptable, because only a little bit of curvature is necessary.
