I have Riven, Broken Blade on star Star Spring I play Blitzcrank, Impassive to Star Spring can I sequence the chain with Blitzcrank, Impassive ability then move a unit resolve the showdown before the star Star Spring resolves or am I forced to push back after

No — you can pick the order of your two triggers (you control both, so Rule 383.3.d lets you choose), but the combat staged by the hooked unit can't begin until the chain is empty, so Star Spring's trigger always finishes resolving before any showdown starts. Combat only opens when the chain is clear (Rule 462), it doesn't jump in mid-chain — same as the Here to Help/Blitzcrank case where the pulled unit's combat waits out the whole chain.

The order choice is still real, because it changes what's on the board when Star Spring resolves: put Star Spring's trigger on the chain last and it resolves first (LIFO), so you pick and send a unit to base before the enemy unit gets hauled in. Put Blitzcrank's hook last and it resolves first — then you see the hooked unit sitting at Star Spring while you resolve the push. Either way the target for Star Spring's move is locked in at finalization, so you can't wait to see the hook result before choosing.

You're also not forced to push at all — "they may" is the first part of the effect, so you decline it at finalization and it's removed from the chain (Rule 383.3.a.2); nothing happens and no unit goes back to base. One heads-up: since you already control Star Spring, Blitzcrank doesn't contest it — the hooked enemy unit does, so you're the Defender in that combat, not the Attacker.