WASHINGTON — Before Donald Trump became president and after, his exceedingly complex and voluminous tax returns came under regular scrutiny by the Internal Revenue Service. The number of agents assigned to the audit team: one. After Trump left office, the IRS said it was beefing up the audit team, to three. The tax agency itself acknowledged that it was still overwhelmed by the complexity of Trump’s finances and the resistance mounted by the former president and his sophisticated army of accountants and lawyers, which included a former IRS chief counsel and raised questions early last year about why even three...