So, WI the Mexican-American War hadn't ocurred in 1846, or would such a conflict between the US & Mexico have been inevitable at some point during that period ?
The way I see it, conflict, while not inevitable, is fairly likely. The US had three main avenues of expansion: Mexico (including Texas), the Caribbean, and Canada. Canada is unlikely. It would bring us once more into conflict with Britain, which is a
bad thing, especially given finally warming relations. The Caribbean was still largely colonial, meaning it's a bad avenue for expansion. A lot of Spanish colonies, a lot of British colonies... Haiti was a black republic (obviously the Southerners wouldn't exactly like it). I
think the Dominican Republic was the only other free Caribbean nation at the time, but that's a relatively small expansion, anyway, and would hardly distract the US from long.
The last possible avenue of expansion, Mexico, is meanwhile fracturing (Texas, Rio Grande, Yucatan...), has an economy in the shithole, and didn't have major European connections like the Caribbean and Canada. So, considering the options... tiny Dominican Republic, black Haiti, European colonies, or large, resource-rich but vulnerable Mexico, the choice is clear. So the two ways I can see the US avoiding war with Mexico is if they either had a cultural shift and wasn't so expansionist (unlikely, but definitely possible) or Mexico just caved and gave in to giving up its claims on the Republic of Mexico (very unlikely... Mexico isn't just going to give up large pieces of land), unless we move the POD back a bit (no independence from Spain, for example).
Even if there isn't an actual war, we'll still see the US gobbling up Mexican breakaway republics (Texas and California, maybe even places like Rio Grande). Not an actual Mexican-American War, but it'll have the same basic results.
Sorry if this doesn't answer your question of effects, but I hope I've emphasized enough how difficult it is to prevent the Mexican-American War, in some form, from occurring.