Question 1 - Middle (safe route)
Placing Js in the front row will require you to hit 2 pairs in the middle row in order to avoid fouling this hand. Even if you hit trip 8's in the middle, you will still be required to hit another 3 or full house in the back row.
Placing Js in the back row lowers your chance of catching a full house in the back (since there is already Jc up front).
Question 2 Back (aggressive route)
Although placing Ah in the back row puts you in danger of fouling, you still have close to half the deck on 13th street to make your gamble for the 10-point royalty(quad Aces in the back) and maybe an extra 4-point royalty (straight in the middle) well worth it. Any 8, 9, T, J, Q or K will give you either a pair or a straight in the middle row.