As a bit of a tangent to 2)
Certainly using visualisation as practice has some evidence (especially high-fidelity visualisation increasing performance at comparable rates to actual practice; one course I’ve been to advocated for the PETLEPP model in the context of medical procedures/simulation) - in this sense it may help achieving an endeavor but 1. It’s got nothing (much) to do with positive visualisation and 2. It feels like its moving the goal-posts by interpreting the ‘endeavor’ as ‘performing better’.
I’ve definitely also heard people discussing positive and negative visualisation as tools for emotional stabilisation and motivation—although the more persuasive (read: not sounding like new age/low brow self help BS) usually favour using both together or just negative visualisation - see gjm’s and Unnamed’s posts