Vaccines may or may not produce complete neutralization. This is extraordinarily variable, and depends on both the virus as well as the immune system of each vaccine recipient. Clearly vaccination against COVID-19 appears to offer imperfect protection against infection. However, that's actually the case for most vaccines, it just hasn't been as carefully studied previously since for many other diseases mild infections in vaccinated individuals were not significantly studied because they were clinically irrelevant.
To answer your second question there isn't a magic threshold unless you're going to start measuring single monoclonal antibody activity against specific virus in-vitro, since there is so much variability in host response. There is some literature on effective antibody concentrations against SARS-CoV-2 and you're welcome to peruse pubmed.gov for it, but that's highly specialized literature.
To answer your third question completely I would have to explain to you how you develop B-cell immunity. In short, you form a large panel of immature B-cell receptors through targeted mutation. When antigen presenting cells encounter B-cells in the lymph nodes, the ones that happen to have some binding activity (under specific circumstances) are activated. These then undergo affinity maturation where further mutations refine the sequence to maximize binding efficiency. These cells are then precursors to plasma cells that produce antibody and memory cells that remain quiescent till next infection. The spike protein has multiple antigenic sites, and there can be many linear or conformational epitopes that can be recognized differently by numerous B-cell receptors (out of a random pool). Thus EVERY induced immunity is by definition polyclonal (meaning comprised of antibody with a different binding site sequence that result from different B-cell receptor sequences). Monoclonal antibodies are made by inducing polyclonal immunity, then isolating the B-cells individually to produce a large number of monoclonal lines, and then these are tested against the original antigen and the best candidates are selected through sequential rounds of affinity testing. The best candidates then undergo transformation into immortal cell lines which can be scaled up indefinitely under laboratory conditions to produce what we know as antibody-mediated biological drugs.