Suppose you have a top heavy DB and 401(k) plan that are general tested (15 participants in each plan). Also, suppose one participant was full time but dropped to part time (under 1,000 hrs). The Top heavy minimum is provided in the 401(k) plan. Since he is employed on the last day of the year he gets the 5% top heavy minimum. Does he then get bumped up to the 7.5% gateway because of the DB/DC plans? Assume HCEs have high enough allocation rates such that 7.5% is required.

He would not otherwise be entitled to a benefit accrual or top heavy minimum in the DB (if the TH minimum were provided there) because he works less than 1,000 hrs.

Now suppose he only ever became eligible for the salary deferral part of the 401(k) plan but was never eligible for non-elective, match or entry to the DB plan. Would he receive a 5% TH minimum in the 401(k) plan or a 3% TH minimum.

Yes, 7.5%.

3%, but then gateway (of course could be OE and not needed).

Yes for gateway and remember in order to use the DC side to facilitate the DB, the allocation must be "uniform".

Your last comment only applies as a rule under 401(a)(26) in an offset plan situation. Benefits must be uniform in the DC plan to allow a person's benefit in the DB plan to be considered without regard to the offset. There is no mention that this is an offset situation.