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Twenty three words put S first and T third, and every single one has a vowel in the second box. There are no other options.
A and I tie at seven words each, then E on four, U on three and O on two. All five vowels appear there and nothing else does, so the second box is a choice between vowels rather than a choice from the whole alphabet.
Several of these are narrow. SATEM, SOTOL, SUTOR and SUTTA are real but uncommon, and whether a given word game accepts them depends on the list that game runs on. SATIN, SITES and SETUP are the safe ones to lead with.
The fourth box is the opposite of the second. Ten different letters turn up there and E leads on only five, so it narrows very little. Spend your guesses on the second and fifth boxes instead.
Yes, across all twenty three words. That is unusual and it makes the second box the cheapest one to test on this pattern.
Only in the last box, and both are ordinary words. SITUS and SITUP sit on the same stem, so the SIT branch can swallow several turns.
As a noun it is one word and that is the form on this list. The two word verb phrase would not fit five letters anyway.