The airport branch isn't at 90 degrees to the GWML - it's more like 75-80. Given the proximity of New Stockley Bridge and the gradients involved to get a West Curve to meet the Airport Branch, it just isn't practical/feasible. Besides which I was of the understanding that the ponds were declared a site of special scientific interest during the drafting of the crossrail bill, precluding some of the proposals for how to adapt Airport Jn.
The new junction at Langley will have the room to be Grade-Seperated, the route is mostly free from obstacles such as housing etc, it really is a no-brainer. Through services via Heathrow would also most likely provide enough trains west of Airport Jn to justify Crossrail completely replacing GW Thames Valley services on the relief lines. Your 4 HX paths could be reused by GW to run 2tph PAD - fast to Heathrow, SLO, MAI, TWY, RDG and all stations to Oxford; 1tph PAD - fast to Heathrow, SLO - RDG - (DID) - OXF (as per current 1Dxx services) and 1tph fast to Heathrow, MAI - RDG and semi-fast to Westbury (similar to the hourly fast Bedwyns today)