Can you smite with a thrown javelin?

By RAW, Throwing a Javelin is a MWE, and Smite only requires a MWE, so it works. The same thing does NOT work with the SCAG cantrips, which require a [Melee Attack with a Weapon]. To be specific, you must use a [Weapon], so no [Unarmed].

Does smite crit?

Well, no. You can only expend one spell slot per attack, and you can’t count on criticals happening, so if you wait until you crit to burn smites, you’ll end up missing opportunites where you should be smiting.

Is a nat 20 double damage?

In D&D 3rd and 4th edition, a natural 20 on an attack roll may trigger a critical hit. In 3rd edition, a second attack roll must be made to confirm the critical; if this is successful, the attack deals double damage.

Can you crit with inflict wounds?

Yes! Attack rolls for spells are attack rolls, per the page you cite (PHB p. 194 ), so spell attacks can score a critical hit. Whether you’re striking with a melee weapon, firing a weapon at range, or making an attack roll as part of a spell, an attack has a simple structure.

Can inflict wounds kill?

Inflict Wounds, regardless of its outward manifestations, does drain life energy from the subject. Repeated application will kill, and horribly. When the effect is apparent, bleeding, pustules, bone-crushing deformation, the pain seems to center on the effect area.

Can you twin inflict wounds?

Twinned and Quicken can be used on the same turn, but not the same spell. However, you are still restricted by the Bonus Action casting rules, so you cannot cast Inflict Wounds twice on one turn.

Can you twin spell Healing Word?

No, because the use of Healing Word as a bonus action prevents any other spells beyond cantrips from being cast that same turn. So you could Twin Cure Wounds, or you could Twin Healing Word, but you could not Twin both because you cannot cast both.