Two things that don't make sense to me.

Consider, however, that this kind of "action" genre demands that the filmmaker compress some action, especially when things like return trips to previous settings are needed. These are leaps (appropriate here, I suppose) of logic and imagination that the audience automatically makes. The success of such cuts, however, is dependent on story and genre. In such a film as ISM, this works. Look at adventure fiction and you see the same "cutting" used by many action writers like Robert E. Howard and others.

Yes, the canyon part was most infuriating, like it really took great thought to try to move this stick, which was already part way across the chasm, along to the end.

A similarly annoying part for me is how he kept antagonising that spider. He kept claiming that it wants his cake when clearly it had no interest in it. He eventually kills it because of this story. He should've just left it be.

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the second time he tries to cross the chasm, he hooks the top of the paint can
and then walks around the edge of it...I assume this is how he got back down the
first time.

His whole adventure in the basement was transitional. Upstairs his shrinking made him less of a man, and he lost his identity and importance. Once he became small enough to pass through the screen and go outside he regained an identity as part of the universe; his reduction in size ultimately freed him from the prison of self-importance.

The time he spent in the basement represented the transition between these two states of existance. His mind was working to catch up with the new possibilites offered by his reduced size. When he first sees the obstacle he is thinking in terms of what he can't do; but as he contemplates the situation he realizes that while his size has turned this household setting into an obstacle, it has also opened up an opportunity for overcoming it.