HELL NO.
it will remain what it is. the content /prize already settle what it will be once the loot box drop into your account.
so even you keep it unopen until Sombra come out, all the box from pre-sombra event will remain same. only new loot box drop in post-sombra event then will have chance get new skin (if they already roll out sombra as hero)
and halloween item you can buy with credit that's true, but then you ONLY can buy during the halloween event. after this you cannot purchase anymore.

sound like your monitor cant run the resolution you set game to.. what your res and pc spec and monitor model..