1 and 3
PHB, pg. 196
Roll all of the attack's damage dice twice and add them together. Then add any relevant modifiers as normal.
Crits only double the dice rolled, not the modifiers. It also doubles any and all dice applied to the damage. If a spell or ability added any dice to the attack damage, you roll those twice as well. So, a +1 dagger does 2d4 + 1 damage on crit. Another example, Eldritch Blast hitting a target affected by Hex does 1d10 + 1d6 normally and 2d10 + 2d6 on crit, plus any relevant modifiers such as Agonizing Blast.
When damage is listed for any weapon, attack, spell, etc it applies to all of the dice and modifiers that come before the damage type. Thus, a weapon that does 1d6+2 slashing does all slashing. A weapon that does 1d6 slashing +2 necrotic will do 1d6 slashing damage and 2 necrotic. If it were listed as 1d6 +2 necrotic, it will do all necrotic damage. Damage types are never assumed, they are explicit.