Yes, you are pretty much right, especially about the first part of the fourth line. The stress would fall on first syllable of "Beauty," making that dactylic (I think!). Keats must have done that intentionally because he wanted to emphasize the word "beauty" and because he occasionally departed from the dominant meter just for the sake of variety. In "Ode to a Nightingale" he has a line that reads: "Fast fading violets, covered up in leaves." I don't know what kind of meter that is, but it isn't iambic pentameter. It even has eleven syllables. It seems to me that that line uses syncopation. You can probably find other lines in "Ode to a Grecian Urn" that are not iambic pentameter. There is nothing wrong with that. A poem can be said to be in iambic pentameter if the dominant meter is in iambic pentameter. You can find departures from iambic pentameter in Shakespeare's sonnets too, and certainly plenty in his plays.

Your analysis is absolutely correct and very well explained.

Iambic pentameter, the most common of all rhythmic patterns in Western English poetry, is a pattern of five pairs of syllables with the accent on the second syllable in each of those pairs. The first three lines follow this pattern. The fourth line of the excerpt you presented is the exception. It would be possible to render the accents differently than the way you suggest - "Beau-ty/ is truth/ truth beau-ty/ that is all." This changes the analysis of why the fourth line doesn't follow the iambic pentameter rhythm but still supports your conclusion that this line is the exception to the pattern.

The fifth line reverts to following the expected format for iambic pentameter strictly.