Concerning Luke 17:21 and its syntax (word order):
οὐδὲ ἐροῦσιν Ἰδοὺ ὧδε ἤ Ἐκεῖ· ἰδοὺ γὰρ ἡ βασιλεία τοῦ θεοῦ ἐντὸς ὑμῶν ἐστίν (NA27).
The NIV reads: "nor will people say, 'Here it is, ' or 'There it is,' because the kingdom of God is within you."
The margin offers the alternative: "Or among."
Ralph Earle writes: "Some commentators and translators prefer 'in your midst' (NASB). But the Greek preposition here is not EN, 'in'; it is ENTOS (only here and Matt. 23:26), which means 'within.' Alfred Plummer discusses both translations. His conclusion is that if 'within you' is adopted, the meaning will be, 'Instead of being something externally visible, the Kingdom is essentially spiritual; it is in your hearts, if you possess it at all' (p. 406). It would seem that this is what Jesus meant" (Earle, Word Meanings, page 72).
Louw-Nida, however, lists the semantic domains of ἐντὸς as follows:
(b) what is inside
This reference work then notes:
83.9 "ἐντὸς: ἡ βασιλεία τοῦ θεοῦ ἐντὸς ὑμῶν ἐστίν 'the kingdom of God is among you' or ' . . . in your midst' Lk 17:21. For another interpretation of ἐντὸς in Lk 17:21, see discussion at 26.1."
83.17 "ἐντὸς: pertaining to being within an area - 'the contents of, that which is inside.' καθάρισον πρῶτον τὸ ἐντὸς τοῦ ποτηρίου 'clean what is inside the cup first' Mt 23:26. It is also possible, though not probable, to understand ἐντὸς in Mt 23:26 as the inside surface itself rather than the contents."
26.1 "In Lk 17:21 the phrase ἐντὸς ὑμῶν in the statement IDOU GAR hH BASILEIA TOU QEOU ἐντὸς ὑμῶν ESTIN 'look, God's reign is within you' may constitute a reference to the same inner being designated by the phrases hO ESW and hO EN TW XRUPTWi. On this basis some scholars have suggested that the phrase ENTOS hUMWN can be interpreted as a potentiality for participation and hence be translated 'within your grasp,' but it is more likely that one should understand the phrase ENTOS hUMWN in Lk 17:21 as a spacial relationship, for example, 'in your midst' or 'among you' (see 83.9)."
See the full discussion of ENTOS in BDAG, which says in part: "Lk generally avoids ref. to God's reign as a psychological reality. The passage has invited much debate" (BDAG 340-341).