"We must assume that the 'full sequence' of reel spins is completed when once the real jackpot (3rd line payout) is won."

Not true, the jackpot could pay out on the 1st pull, in fact it could pay out 5 times on the 1st 5 pulls.

You are like talking to the wall!

EVERY spin has the same chance of coming up on a jackpot as the previous one does. If it does not have this chance its illegal in NV.
Its called an RNG.

The hold is based on 3 coins played.
2 coins played will skew the odds to the house's favor because you have eliminated the chance of the jackpot.

When you see 98% payback on a poker machine its based on 5 coins played. Look at the difference paid for a Royal Flush with 4 coins played and 5 coins played. Its the only spot on the pay table that is out of sync with the rest of the escalating scale. You must hit the Royal with 5 coins played to obtain the 98% payback.