If Saruman could be trusted then the journey could have been down the west side of the Misty Mountains (no Moria and obviously no bird-spies of Saruman) and into Rohan, then Gondor and over into Mordor with little risk before crossing the Anduin. Isengard was originally specifically built to block the Gap of Rohan to Sauron's forces and help keep him at bay to the East.
Saruman, Gandalf and Aragorn would have been quite formidable as a team using the Isengard palantier to distract Sauron and watch his activities.
Generally speaking, having Saruman on-side would have made the whole enterprise an order of magnitude easier but the specific ability to avoid Moria was probably the main reason Gandalf wanted to make sure Saruman was going to finally act against Sauron. So off he went to answer the summons...
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