Regarding the statute of limitations and filing under extension, see section 25.6.1.6.15 of the Internal Revenue Manual: https://www.irs.gov/irm/part25/irm_25-006-001r
The statute of limitations clock begins on the filing date (IRM 25.6.1.9.2). In part, IRM 25.6.1.6.15 says:
a. Early-filed Tax return. A tax return that is filed before the due date (determined without regard to any extension of time for filing) is considered filed on the due date.
Note:
This rule does not apply to returns concerning (1) the withholding tax on nonresident aliens and foreign corporations, (2) the Federal Insurance Contributions Act, and (3) the Collection of Income Tax at Source on Wages. The returns for (2) and (3) for any period within a calendar year are treated as filed on April 15 following the end of the calendar year. IRC § 6501(b)(2).
b. Extended Due Date. An extended due date is not treated like the regular due date. The filing is not considered early when the taxpayer does not use the full extension period. A timely return received before the end of the extension period (including postponement for disaster relief and combat zone) is considered filed on the received date, not on the extended due date. [Emphasis mine.]
