In US law, there was, as far as the question indicates, no probable cause to search her phone at all, Therefore (unless there is some cause not mentioned in the question), any such search is illegal, and any evidence found in such a search, or that is found as an indirect result of such a search (pointers toward it are found in the search, and followed) would not be admissible in any criminal case against Alice.
In the case of Bob, if his friends and family approach the police or other authority with
a vague suspicion that Bob might be involved in the creation of illegal content
That will probably not constitute probable cause for an arrest of Bob or a search warrant for his phone. Unless the accusation does prove to constitute probable cause, any evidence found during such a search would not be admissible against Bob in a criminal case.
In practice, most US police would not undertake either search without better evidence than is described in the question. But some police will overstep the lines, which is what the US exclusionary rule is for.
Legal procedure does not as far as I know make a distinction between "exploratory" and "confirmatory" evidence. Instead, evidence is either admissible or not. The rules for when evidence is admissible are quite complex, and vary by jurisdiction. Some of them are more traditional than logical, and some of them are addressed to particular problems that have arisen in particular circumstances.
But the US Fourth Amendment protections against unreasonable searches, and the requirement of probable cause before search or arrest warrants are issued, serves some of the same purpose.
Other countries have different rules, but many of them restrict the authorities to some extent from making arbitrary searches with no initial evidence.
Response to the Revised Question
As the question has been edited, there seems to be fairly clear probable cause to search Alice's phone, and if clear evidence of "illegal pornographic content" presumably actually child pornography, as no other kind is illegal simply to posses) is found, she can be brought to trial and perhaps convicted. The mere "suspicion" of Alice's "friends and relatives" would add little and mi8ght well not even be admissible. The facts, if any, on which those suspicions are based might be admissible, one cannot tell from the summary in the question.
The case against Bob, however, remains weak. Indeed there still seems to be no probable cause either to arrest Bob nor to se3arch his phone, and the results of any search that was done would not be admissible. Probably none would be done without more evidence.
The OP wrote:
Thus, although the situation looks grim for both, since the evidence against Bob is confirmatory, it might be considered stronger.
Not so, the case against Bob is weaker, indeed so weak that an arrest would be unlikely, and if one were made, the case would likely be dismissed before going to trial, assuming no more evidence than was included inn the question. The evidence prior to the search seems to consist only of vague suspicion not supported by any actual evidence, and so there is nothing to confirm, and no valid search would occur.
That suspicion of Bob came before the search, and the search is thus "confirmatory" is not relevant. The question is, what evidence against each defendant is admissible, and does the totality of the admissible evidence amount to "proof beyond a reasonable doubt" no matter what order it was discovered in, or what idea was in the minds of the investigators, provided that they were acting lawfully so that their findings are admissible.
