I think you miss the point.
Actually, I think it is you who are confused.
There's clearly a gray area here where the use of campaign funds is concerned for the POTUS to pay his legal expenses as derived from his presidential campaign.
The "personal use" clause should prohibit this expenditure at the very least because the expenses were derived from an on-going investigation - an investigation I contend would NOT have happened had there not be probably cause and, thus, legal expenditures would NOT have existed if Donald J. Trump, Sr not ran for public office or even won the presidency. The fact that he hired his own legal team well before charges were even levied against him should tell you something. He didn't do it to fight off defamatory claims against his character and he shouldn't be doing it to pay off legal expenses he incurred from any law suit levied against him from his involvement with Trump Org. as a private citizen.
You cite the Personal Use clause without understanding. From your own citation:
Personal use includes but is not limited to the use of funds in a campaign account for any item listed in paragraphs (g)(1)(i)(A) through (J) of this section:
https://www.law.cornell.edu/definit...itle:11:Chapter:I:Subchapter:A:Part:113:113.2
That section lists items for which campaign funds are specifically identified as personal use and thereby prohibited.
However, the next section applies to the OP issue:
(ii) The Commission will determine, on a case-by-case basis, whether other uses of funds in a campaign account fulfill a commitment, obligation or expense that would exist irrespective of the candidate's campaign or duties as a Federal officeholder, and therefore are personal use. Examples of such other uses include:
(A) Legal expenses;
https://www.law.cornell.edu/definit...itle:11:Chapter:I:Subchapter:A:Part:113:113.2
The question every citizen should rightfully be asking is what justification did the FEC use to authorize these legal payments? Did they agree simply because Donald J. Trump, Sr. is now POTUS and it believes said legal fees were closely tied to his campaign? If so, did it believe Trump would have incurred legal troubles whether he won the presidency or not? If the answer is YES, then this does lend itself to question what's really going on.
The answer is as clear as the cause of the investigation...allegations that the Trump campaign colluded with a foreign power to influence the election process in order to win the election.
Now you make reference to legal issues you don't appear to understand.
1. Probable Cause. This is a term most used in the context of arrest and search warrants. It is based on legal interpretations of the Fourth Amendments restrictions on search and seizure.
https://www.law.cornell.edu/wex/probable_cause
2. Complaint. A criminal investigation typically starts as the result of some complaint (via victim, witness, or police officer) of a crime that has been committed. The purpose of the investigation is to determine if the complaint has a factual basis sufficient to lead to actual charges. A crime of some kind must have been committed, and the investigation is initiated to try to identify the perpetrators and bring them to justice.
Probable cause has nothing to do with the establishment of the Trump investigation.
There have been allegations (a complaint) of collusion. While there is yet to be an identified crime, Congress has leeway when considering possible impeachment proceedings to demand an investigation. They may conduct it, or request a special counsel.
So, the facts are:
a. The issue being investigated is based on alleged campaign malfeasance of some kind. Thus, under the Personal Use clause
you cited, funds from the campaign may be used for legal costs.
b. There is no crime, and this investigation is seeking a crime rather than investigating one. Thus..."mud-slinging" IMO.
c. The rest of your (honestly confusing) rambles about "if he had not run...levied as a private citizen" have nothing to do with the issue. The investigation exists because he ran for office. Campaign funds are raised to cover costs of running for office. If legal issues results as a direct result of the campaign, campaign funds may be used to cover legal costs.
THAT is the logical trail the Commission would follow to allow this usage.