Question Mercedes ml350 2014 3 across

My kids occassionally ride in my husband's car and I just checked and my oldest(8 yo 50") outgrew the diono rxt in there. We have to have 3 in there to divide and conquer activities (there's only a 2nd row), right now all are diono rxts. My 6 yo (47.75 in and 50 lbs) still has about 1.5 in till he outgrows the diono by height. Since my 4yo and my youngest (10 mo and only rides in my car now because that's where the rf seat is) also need seats in his car on occasion, I'm hesitant about moving to 2 bob boosters and having to totally switch seats instead of just rear face one in a pinch.

I'm looking for a longer term solution as my kids grow and get one hbb with the two dionos now and then as my 6 and 4 yo grow (& mature) to then have two hbb and one diono.

What hbb is narrow enough to be outboard next to 2 dionos?
What 2 hbb are narrow enough to be next to one diono?
Hopefully there's an install scenario that allows for hbb to buckle norton the same side as the diono. My kids seem to have this knack for unbuckling anything nearby and have had kids accidentally unbuckle uninstall 5pt seats too many times to count.

Some of the more narrow boosters include the Britax Parkway and the Graco Turbobooster. At 50", your child may already be too tall for the Turbobooster. The Parkway is a good 2-3" taller than the turbobooster. You may also want to consider a no-back booster for an 8yo, especially for occasional use. In that case you can use something narrow, like a Harmony youth booster or even a backless turbobooster. Just make sure there is a headrest in the vehicle in the spot where he will be sitting.

For accidental unbuckling of the RXT, you can turn the bottom part of the buckle (female end) around 180* before buckling and finishing the install. No one will be able to accidentally uninstall it! In fact, when you do try to uninstall purposely, it is very difficult and painful on the fingers!