Not international observers but only Maduro's puppets. There are not international observers in Venezuela, as only 'accompaniment' is permitted, and the govt hand-picks who it accredits. One of the "international observers" that were actually acting as acompaniment was a partner of Maduro who received huge contracts from the regime to import food for the food program used to control the population. [Source]
Your "international observers" were already debunked:
The CEELA doesn't have a website on their own an the information on them is almost non-existant so you have to dig a bit to find that they are not an independent group but one clearly in line with the government (here an article from 2008 about how the CEELA came to be). There is no organizational charter, there is no info on their funding, how they choose their members. The same problem with the funding goes for the Caribbean Nations report: if you anything about the CARICOM you will know their members profited from the Chávez mandate and in turn they give the Venezuelan elections a semblance of legitimacy. All in exchange for oil. Their report is also hosted on Venezuelanalysis and it's not even digitally signed.
Lastly the last report from the African Nations have the same problem, they are not independent group for monitoring elections but put together ad hoc by the Venezuelan government. They don't even have a mission statement, they don't even have a website, their report had to be hosted in the website of a Venezuelan embassy. In turn I can show you that the bar to be an "independent" observer is not that high: Gassan Salama, a government contractor that sells the CLAP boxes, was one of those "observers" for the presidential elections
The UN were not invited as observers but as accompaniment. The opposition was not allowed to participate in the election after the Maduro's Supreme Court barred the opposition coalition from the presidential election. Source (in Spanish)
Maduro's Supreme Court also banned the most prominent opposition leaders from the presidential election. Source
The opposition didn't boycott the election. The vast majority of the population didn't vote because their political leaders were not candidates. There was not an election but an imposition from Maduro. He imposed himself and declared himself president in a illegal and illegitime move against the constitution and against the will of the people.
Participation in Venezuela’s controversial presidential election on May of 2018 was at 32.3 percent by 6 p.m. as most polling stations began closing, according to an election board source. [Source] Only 6,6 million would have participated in the election and only 4,4 million would have voted for Maduro.
In case your wonder if the current interim president was elected or not, more than 14 million of people elected the National Assembly in the last parliamentary elections that took place on December of 2015, as voter turnout was estimated in 74,25%. [Source]