Prepositions – Why is ‘To’ Not Used Before ‘Home’?

Definitely not "in" - "to" is the correct word. Unless it's significant that their "home" (as opposed to their "home country") is somewhere else, just "Welcome home!" is the right phrase. If you do want to separate "home" from "home country" then something like "xx welcomes you back!" could do it where xx is the country.

Home is a preposition according to modern grammars such as Oxford Modern English Grammar (Aarts 2011) or The Cambridge Grammar of the English Language (Huddleston & Pullum 2002).

According to such grammars, although prepositions often—prototypically in fact—take noun phrase complements, this is not an essential feature of the word category. In the same way that verbs that don't take objects are still verbs, prepositions that don't take objects are still prepositions.

First of all, notice that home patterns like other preposition phrases in the Original Poster's example, as a complement of the noun way:

• I can find my way out of the building.
• I can find my way out.
• I can find my way through.
• I can find my way through the forest.
• I can find my way home.

In addition home can be modified by the specialised adverbs right and straight, which modify prepositions but not usually adverbs:

• Go straight home.
• Go straight past the station
• I have to go right home.
• Go right through the forest.

In addition, home cannot be modified by the adverb very which freely modifies adverbs (at least those with a gradable meaning) but not prepositions (even those which are gradable):

• *I went very home. (ungrammatical)
• *I went very past the station. (ungrammatical)
• I went very slowly.

Unlike adverbs, prepositions such as home can function as locative complements of the verb BE:

• Are you in the building?
• Are you home yet?
• *Are you locally? (ungrammatical)

Unlike adverbs , preposition phrases commonly modify nouns:

• people home alone
• the man in a hat
• *the locally shop (ungrammatical noun phrase)
• *the shop locally (ungrammatical noun phrase)
• *the beautifully woman (ungrammatical noun phrase)
• *the woman beautifully (ungrammatical noun phrase)

Thus the evidence would seem to suggest that home is a preposition and not an adverb.