Again, imagine the uproar if, say, Romania voted against extension effectively booting the UK out on March 29th.!

From memory, one of the amendments proposed for the discussion on Tuesday 15th was that the UK should ratify the Withdrawal Agreement, subject to writing in unilaterally their own end date to the Backstop! That amendment was not proceeded with. Even before considering A.50 extension, I could imagine that attempting to modify the WA would have caused several of the 27 to reject the WA in the EP.

I found the opening contribution of Sir Geoffery Cox, Attorney General enlightening [top law adviser to the UK government]. I was surprised that 432 "lawmakers" chose to ignore his advice.

Orderly exit from the European Union would always require a withdrawal agreement along these lines. No alternative option now being canvassed in the House would not require the withdrawal agreement and now the backstop. Let us be clear: whatever solution may be fashioned if this motion and deal are defeated, this withdrawal agreement will have to return in much the same form and with much the same content. Therefore, there is no serious or credible objection that has been advanced by any party to the withdrawal agreement.

My emphasis added

It is useful to know that one government advisor has read Article 50, and understood the process, its inflexibility and consequences.

The complete official record of his contribution is here but more difficult to follow because of all the interruptions and his Shakespearean delivery.