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A very short response to the question about the nuclear instabilities of
Tc and Pm imvolves 'instability with respect to what?'  As in ordinary
chemical changes, one must consider not only the structure of some
collection of nuclei and electrons (i.e. a specific molecule), but also
alternative arrangements that might be more stable.  It is not so much
that Tc and Pm isotopes are unstable, but that there are alternate
arrangements, lower in energy, that can be reached relatively easily. 
The beta-decay products are more stable.  (Perhaps somewhat analogously
is the question about why doesn't cyclic tetraoxygen exist as a stable
molecule?  It's got a great Lewis structure!  The answer is that two
di-oxygens are 'more stable' and easily formed from the cyclic tetramer.)

A deeper answer lies in nuclear forces, nuclear structure, and the shape
of the mass-energy surface (or binding energy curve).  It furthermore
demonstrates that all nuclei above A~140 are unstable towards alpha
decay (but don't necessarily have an easy time pulling it off) and that
nuclei of even lower mass number are unstable with respect to
heavy-cluster (e.g. Ne-22) emission.  There's no mystery about Tc and Pm
instability.  It's just that a comprehensible answer is long.

Paul J. Karol
Nuclear Chemist
(also unstable)