Substitute for solder wire and flux
For normal types of electronics work, not really. Solder is an alloy that melts at a low enough temperature that it doesn’t damage components when it’s applied, and it adheres well to copper and brass (hence its use in plumbing.)
If you were thinking about an object with lead to be recycled for solder, don’t. For one thing, pure lead melts at too high a temperature. Solder is a tin-lead alloy or tin-silver-copper alloy for lead-free.
Same story for silver: melting point for the unalloyed metal is too high for soldering. Silver can be used for flame brazing though.
Why bother, anyway? Lead-free solder is widely available (it’s mandatory for plumbing), so working with it poses no health risk other than the flux fumes. Even leaded solder is fairly safe, just wash your hands afterward.
As far as flux, rosin vaporizes when it’s heated and it’s not so great to breathe, use ventilation to draw it away. To answer your second question, most mild acids can be used for flux, but that doesn’t make them any better than rosin (in fact they're probably more harsh.) For example, when I did Tiffany-style stained glass I often used oxalic acid for flux, that wasn’t so great to breathe either.
MORE: different fluxes have different toxicity. Vapor from 'no-clean' fluxes is worse than rosin types. More here: https://www.bhencke.com/blog/2019/3/15/a-hill-to-die-on
I should note that the Apollo Guidance Computer was built without solder or flux. Instead, the wires were bonded to components by spot-welding. Not sure why they did that, weight saving possibly, or eliminating a possible toxic contaminant from the spacecraft. More here: http://www.righto.com/2019/08/reliable-after-50-years-apollo-guidance.html