Doesn't the first unit completed, 458534, use 5 standard class vehicles which already had 2+2 seating throughout? Taken from a couple of broken up 460 formations. It doesn't create much of a precedent for what they may or may not do once they run out of 460 standard class vehicles and start using converted 'firsts', and gives little or no indication of what they'll do once they start on a proper 458 conversion, which won't be for a while yet, cos none have left yet...
It's perfectly feasible that the seating style won't be standardised across the class - we could end up with a few sets with arms ex 460, but the majority without them...