Ok, if he had time to move them en masse.. well then why did he need to travel back in time?
The infiltrators thing, ok no real indication there in anything official but it makes sense, I can buy that. But the large Venatori force in Redcliffe? Well that one I can't really latch onto I'm afraid. It would take a ton of time to secretly move a large force in small enough numbers not to be noticed, but even more troublesome is that the cut scenes in the game don't support that notion, they contradict it in fact.
When your agents kill Alexius's men, it doesn't take a huge army to overwhelm them.. because there just aren't that many of them there. I guess one could assume a huge force of Venatori elsewhere in the castle or in Redcliffe that are never mentioned or shown - but if such is the case then your advisors probably would never have signed off on the whole sent agents through to retake the castle plan, because if it were being held by a huge force of Venatori the effort wouldn't have had a chance to succeed.
My problem with your central theory, that Alexius couldn't have enough forces to make a difference, is that, when you are tossed into the future, you are explicitly told that he did. He not only had enough forces to make his argument convincing, he had enough to hold Redcliffe against the Inquisition, the Templars, and the Ferelden Army. So whatever he brought, however they got there, it was enough to make good on his promise.
There are several reasons stealing a march on the Inquisition makes sense. It deprives the Inquisition of the magical might to close the Breach (which Alexius needs open for his time magic), it gets him in a position to take over the greatest defensive fortification in Ferelden (which he does), it augments his forces with additional mages (while depriving his opponents of the same), and, perhaps most importantly, Corypheus told him to.
Now, while I absolutely agree that moving large concentrations of foreign troops around the hostile countryside is more than a little handwaved here, it isn't as though it is unique to this situation. DA:I basically exists on the premise that no one notices armies of Venatori and Red Templars moving around and taking over whole provinces and attacking holy sites while manipulating nobility and whatever. Everyone in DA:I that isn't part of the Inquisition is blind, stupid, crazy, or all of the above.