1. The wording might be confusing, apologies! You have it about right. Arachne's pedipalps are largely used for grasping or pinning things, rather than supporting the body or doing any major lifting. It's difficult to quantify because Arachne can differ in size and strength (especially based on gender - female Arachne tend to be larger/stronger), but generally speaking, pedipalps are some degree weaker than the rest of the legs because they're shorter and meant to hold prey in place.

2. Yes, it would be classed as tactile sensitivity, and extends to the whole body, not just the legs. You have it right about the sensitivity to vibrations.

3. Yes! A possible side effect of an Arachne's venom can be to liquefy flesh, but would be better suited to smaller prey (insects, small animals, etc.). Dissolving a whole human would take weeks, but vermin slurpees are absolutely on the table.