A#50: My gut says shove all-in, but let's do the REDi dance first and see what she says....
Reads: The UTG player folds prior to us making our decision, so you might think he's irrelevant. Well, not quite; his range does affect (combinatorically) the range of the MP villain, so we probably should put him on a range. He raises to 4x UTG, so his range is probably pretty tight. Call it 88-AA, AK+, and AQs. When he cbets on the dry J♣-2♦-6♠ flop and then folds to our reraise, we can assume he had something like overcards or the smaller end of his pair range. Call it 88-99, AQ and possibly AK.
So why is this UTG range important? Because it makes it less likely that the MP villain has these cards in his range. Further, when the LAg villain smooths preflop in MP, we can probably rule out hands like TT-AA, and AJs+, because a LAg would probably reraise from the middle of the field with those hands. Therefore, we can put him on hands like 22-77, maybe 88-99, and things like big suited connectors.
The weird call-reraise line on the flop is odd from the LAg. It's possible he's either bluffing, semi-bluffing, or hit a big hand. There are no straights or flush draws on the board. We've mostly ruled out the JJ hand from his range, but there is a slim possibility he has the fishhooks in his range. Two pair is unlikely, as he's not going to play hands like J-2, J-6, or 6-2. So this leaves 22, X-Js, type hands, and maybe some underpair bluffs.
Estimate: We crush the villain's range. Also, there is $4,075 in the pot, and you have $3,400 behind when the MP player shoves. Our current SPR is therefore $3400/$4075 = 0.8.
Decide: We're swimming in value, and SPR says we're fully pot committed.
Implement: Of the the two action choices in the problem statement, All-in is the only one that makes sense when on a Value line and committed.
All-in for now...