YUCK!!  There seems to be no end to the challenging situations one can
encounter under the general topic of "shell cleaning".  Detergents can
do a good job of removing most oils, and it sounds like you have gone
about as far as you can go with that approach.  Unfortunately,
detergents are pretty useless when it comes to dissolving plastics.
The problem here is that the term "plastic" encompasses a huge array
of materials, with widely varying solubility characteristics.  For
example, if you drop a styrofoam coffee cup (a type of plastic) into
a container of a common hydrocarbon solvent like xylene or toluene,
the cup will be gone (completely dissolved) in about 10 seconds.
But, a piece of a plastic milk bottle dropped into the same solvents
will sit there for weeks, with no apparent effect.  So, each kind of
plastic is susceptible to certain solvents, and not to others.  Some
plastics will dissolve in many different solvents, others in only a
few.  So, the crucial question is "what kind of plastic are we
dealing with?" - a question you probably have no way of answering, in
this case.  Therefore, the only thing to do is to identify solvents
that you can get your hands on, and which you know will not damage
the shells themselves, and then try them.  In general, solvents that
do not contain water are not likely to harm the shells.  Aqueous
(water-based) solutions like ammonia, drain cleaners, bleaches,
porcelain cleaners, oven cleaners, detergents, lye, etc. are not
likely to have an effect on plastics, and some of them may contain
substances which are destructive to shells.  Alcohols are non-aqueous
solvents, and will not harm shells, but are unlikely to have much
effect on plastics.  The best bet is a group of solvents known
collectively as hydrocarbons (compounds composed totally or primarily
of hydrogen and carbon).  Most solvents that will dissolve plastics
are members of this group, and virtually none of these solvents will
damage the calcium carbonate matrix or the pigments of shells.  Some
non-aqueous solvents that are commonly available are:  charcoal
lighter, cigarette lighter fluid, coleman lantern fuel, kerosene,
gasoline, carbon tetrachloride (dry cleaning fluid), acetone
(fingernail polish remover), paint thinners, paint removers
(non-aqueous ones only), and mineral spirits.  All of these will
dissolve some plastics.  None of them will dissolve all plastics.
NOTE:  almost all of these substances are FLAMMABLE, and some of them
are HIGHLY FLAMMABLE!  Vapors allowed to collect in an enclosed area
can be EXPLOSIVE.  Also, the vapors of some of them areTOXIC to
breathe.  They should be used only in a well ventilated area,
preferably outdoors, and kept well away from all sources of flame,
heat, and sparks.  That includes electric motors - no ventilation
fans!  Nothing with an on-off switch, not even a light!  Not even a
battery-powered light or fan!  A tiny spark from any such device
could ignite the air-borne vapors of a flammable solvent.  And, some
organic solvents can be absorbed into your body through your skin.
Use appropriate protection, especially rubber gloves.  Even then,
don't immerse your hands in the solvent - use forceps or some other
sort of tool to place shells into, and remove them from the solvent.
Careful - many solvents that dissolve plastics will also dissolve, or
at least soften, various types of rubber (as in gloves).  Eye
protection is recommended, just in  case of a splash.  See what fun
shell cleaning can be?  Not to state the obvious, be careful about
what kind of container you put the solvent into.  Glass is breakable,
but is chemically resistant to organic solvents.  A metal container,
preferably rust-resistant, would be a second choice.  A plastic pail
is good IF the particular plastic the pail is made of is resistant to
the particular solvent you are using!  Most plastic pails are
resistant to most common organic solvents.
If you have access to chemicals other than common household products,
some of best plastic solvents are:  toluene (methylbenzene), xylene
(dimethylbenzene), chloroform, tetrahydrofuran, carbon disulfide,
petroleum ether, ethyl ether, benzene, amyl acetate, dichloroethane,
dioxane, and methoxyethyl acetate.  Most of these will dissolve a
wider range of plastics than the household products listed above.
But again, none of them will dissolve all plastics.  Incidentally,
most of these are also good solvents for paints, varnishes, laquers,
waxes, oils, etc.  The solvents just listed have all the associated
health hazards mentioned above - flammability, explosivity, toxicity,
absorption through the skin, and in some cases to a considerably
greater degree than the previously listed products.  Did I mention
suffocating fumes and possible cancer hazards?  Don't attempt to use
such chemicals unless you are experienced in their use, and have
proper working conditions.  Here, where I work, that means an
approved chemical fume hood, with flashproof lighting and other
electrical components.  I want to provide some information here, but
I don't want to encourage anyone to try something that might endanger
their health - so please excercise extreme caution if you use any such
materials, and if you don't know what precautions are called for with
a particular solvent, don't use it!
If you are not successful in removing the plastic from your shells,
and if you consider the shells in question to be worth the effort,
you could send me a few of the less desireable specimens (perhaps the
ones with the heaviest coating of plastic), and I'll see if I can find
something that will clean them up.  I have at least thirty or so
organic solvents at my disposal that I can try.
Regards,
Paul M.