If it were my vehicle....I would put motorcraft oxygen sensors in the upstream position.
I have no experience with cleaning oxygen sensors.....so I don't know how well it works.
From what I have read.....an oxygen sensor begins to switch more slowly as it ages......
The faster switching speed is only important for the upstream sensors.....as the downstream ones switch slowly anyhow.......due to the fact that they are reacting to how the catalytic converter is working.
I changed my upstream oxygen sensors somwhere after 150K miles.....and I got no change in performance or fuel economy (both were just fine).
Mine had very slight green stains from coolant.
From what you will see in posts here.....the oxygen sensors on the windstar are VERY reliable.
I had to cut or bend the support bracket at the transmission location on my Walker direct fit unit......same reason as you.
Not a big issue......if everything else works out.
You WILL want a flex section at the back of the "Y" pipe.
This is because (as you have noticed) the "Y" pipe is mounted in a rigid fashion to the Motor.....no flex point at all......it moves with the motor/transaxle all as 1 solid unit.
The flex pipe is the ONLY point of flex.....which is why it is importand......otherwise there will be stress on the whole unit from that point.