Can a person ron from a person that sits across or right next to him but the last tile he takes from discard pile are sequence melds? (assume all hands are closed)
Can a person call ron and the last tile he need to complete are the pair? And if this can, can it call ron from everybody or just some people who sit left to him?
Generally yes, but it will depend on which variant you are playing and any special rules. It sounds like you are playing Japanese Mahjong (since you use the term "ron"), in which case you will need to be sure your hand has at least one yaku if you use someone else's tile to win, and obviously you can't be in a "no ten" state.
Precedence rules may also apply if two players are trying to simultaneously win off of the same discard.
Rules vary, but in in most forms of mahjong, it's OK to win on anything that gives your final hand the basic structure of 4 groups of 3 and your pair of two. This can be on sequence groups from anyone (not just the previous player on the left), or on a single wait to form a pair.
Where it gets tricky is that sometimes, the wait might influence some of the winning conditions or the scoring. Hong Kong scoring doesn't care about such considerations. Riichi mahjong cares about fu points for the structure of the last element (minor stuff), as well as for certain winning conditions (yaku) like pinfu that would require an open-ended sequence, or the ankou-based hands that require that the element itself be fully concealed (for sanankou: AAABC XXX YYY ZZ is a hand structure that can win on A/D/Z, but you can't claim the sanankou yaku on Z because the group counts as exposed even as you are finishing. Winning on D is fine, and winning on A simply requires a player to mentally split it into AAA A+BC as opposed to A+AA ABC. Riichi mahjong forces you to score the highest possible score if there can be a variation due to the structure of the hand given any shape or wait.)
(Since I lack rep, small correction on the other answer. If a hand is "no-ten", you aren't in a position of winning at all (as in "not tenpai"). If a hand is "furiten", the hand is in a position of winning ("tenpai"), but is not allowed to win off a discard because of its 3 associated stipulations [persistent: can't win if any of your waits has been discarded by yourself; temporary: can't win off a discard if you let one slide before you get to play again (actually play, call+discard or draw+discard: a turn jumping over you does not absolve this); permanent: can't win off any discard if you let any of your waits slide after calling riichi].