The line beginning “Nilus et” appears to have one extra syllable, the ‘o’ in “alveo”. Then I noticed the following line conveniently begins with a word which begins with a diphthong. So, am I right in thinking the ‘o’ is ill-fated and will be elided interlinearly or does something else happen like combination of the ‘e’ and ‘o’ into a diphthong? I doubt the latter, so I’ve surrounded the final ‘o’ with parentheses to suggest it will disappear metrically when recited.
As it happens, this is matched by Verg. Aen.6.412 … accipit alveo | ingentem Aenean …, of Charon’s ferryboat taking Aeneas on board en route to the underworld (there was a recent thread in the Greek forum on Lucian’s description of the risk of the ferryboat capsizing if overloaded, so ingentem here is pointed), but I don’t think it’s significant that in both cases the next line begins with a vowel. I reckon -veo is collapsed into a single long vowel at line end (synizesis) and that the integrity of the individual verses remains uncompromised.
Spill-over (“hypermetric”) verses are few and far between. Most of them involve elided -que, and tend to be used for dramatic effect. A famous example is Aen.6.602, quos super atra silex iam iam lapsura cadentiqu(e) | imminet adsimilis …. (iam iam lapsura indeed!) Equally famous, and most extraordinary, is the hypermeteric -que at the end of Dido’s furious final speech at Aen.4.629.
OK, bad guess on my part. I knew I’d seen another case of a hypermetric syllable and managed to rediscover where I saw it and it was in fact a line-ending -que. And for the line I asked about, I had failed to check to see if Anderson had a note on it, and lo, he said the same thing you did: synizesis of the “eo” in “alveo”.
So I gather from what you said that the lines of verse can be expected to stand on their own and not be influenced metrically by the syllable beginning the next line. I’ll keep my eye out for further hypermetric lines!