1) Why didn’t Maethros’ ‘repentance’ of wanting the Silmaril last? If his host
was large enough to overcome the people of Sirion and kill almost all of them,
why didn’t he just overpower them and/or imprison them instead?
Overpowering them and imprisoning them would not be possible because the
people of Sirion no doubt resisted. Undoubtedly, they had bows and spears and
swords and spells with which to defend themselves, and when facing an adversary
equipped with such weapons and willing to use them, pacifism is not conducive
to success.
2) Was the Silmaril really the source of the prosperity of the people of
Sirion?
Yes. The Silmaril contains the Light and the power of Hope. With the
Silmaril, as were the people of Ossirand under Lúthien, the people of the
mouths of the Sirion had hope and light and so prosperity.
3) Elwing and the people of Sirion were awfully possessive of the Silmaril
when, technically, Feanor’s sons had a solid claim to the jewel too. Did anyone
really own the Silmarils? If yes, who?
The Silmarils belong to Varda, whose light they contain and derive their
power from.
4) When all was said and done, were the Silmarils a blessing or blight to
Middle-earth?
A blight, not so much because of the Oath of Fëanor but because of Morgoth;
while the Silmarils were in the Iron Crown, Melkor had the power to render
without Hope any adversary who assailed him - even the Valar.
