The life of proton

what does this mean lifetime signify ?

This is the experimental lower limit on the proton lifetime. What it means is just because we havn't seen a proton decay, it doesn't mean that it can't.

What does the proton decompose into?

This probably depends what theoretical model you look at, but it seems that crucially it must conserve the quantity $B-L$, rather than $B$ and $L$ (baryon and lepton number) which are individually conserved in the Standard Model. What scientists tend to search for is the decay of a proton as:

$p^{+} \rightarrow e^{+} \pi^{0}$

and subsequently

$\pi^{0} \rightarrow \gamma \gamma$

Where $\gamma$ is a photon, $e^{+}$ is a positron and $\pi^{0}$ is a neutral pion.

2) Since proton is made up of quarks which though do not individually exist do exist together and have charge, is it possible that under extremely high power electric fields, even proton or electron or neutrons may also get induced charge, i.e. down quarks move to one side and up quarks to one ? Do in their native states they exist as kind of an electrical tripole ?

Perhaps this is more easy to see in terms of neutrons, which are also composed of quarks and are electrically neutral but have a magnetic moment due to the quark content as you mention. This definitely wouldn't happen for electrons as they are not composite particles. It should also be noted that the electron does have a magnetic moment, but this is because it is has intrinsic spin.

Just my opinion, but I think the quoted statement is inconsistent.

An estimate of a half-life as greater than some number is another way of saying that "We looked for decays; if the half-life had been short enough we would have seen some decays with our equipment; we didn't, so it isn't."

OTOH, stability is a theoretical concept: "is there an energetically possible reaction that will lead to the decay in question.

The best example is the isotope Bismuth-209. There was no measurable decay of this isotope: it was listed as stable in most references. However, the alpha-decay of bismuth-209 can easily be shown, from measured isotope masses, to be energetically possible. In 2003, the radiation was detected, and a half-life of 600 yottaseconds was measured for bismuth-209.