Ha you really like ‘bending the rules’, LoL
Those ideas basically fail because you are technically counted as splitting a formation during a move (even though they all count as a single formation for the duration of the assault).
Rules lawyers would argue that this split occurs on disembarking part of the formation, even though you might argue that the Gretchen are still in 5cm from other units inside the Landa, killing them as soon as the Gretchen hit the ground (freed from the yoke of Orkish oppression they scarper pronto
).
However they would definitely die when the Landa disengaged, and re-embarking as part of the consolidation (assuming they won) would prevent the Landa from disengaging.
This is an interesting twist that I have, fortunately, never seen this attempted in a game, otherwise I would probably have cried ‘foul’, that you may not voluntarily split a formation in this manner (you may leave units behind if there is not enough transport to carry them, a different question). However I cannot find any FAQ that states this although I know splitting the formation was discussed in the past. ****
That said, using only a few units like this also risks losing the assault resolution, and consequently both the Landa and its contents, whereas if they have disembarked, the broken Orks could still withdraw.
**** Dave, should a prohibition to voluntarily splitting a formation be added to the FAQ??