Pilot on maintenance flight reported main gear not locked when extended resulting in a divert and two blown tires upon landing.

Narrative

The flight was briefed to launch from ZZZ; perform a limited set of systems tasks and exercise the aircraft in the local area and return to ZZZ. Weather was CAVU with winds light & variable.A normal take-off was executed and shortly thereafter an alternate gear extension was attempted. The normal wind noise was heard and subsequent aircraft drag was experienced as if the gear extended; however; only the nose gear was indicating green. The two main gear lights were extinguished and there was a red light in the gear handle. The handle was moved to the down position with no change. The handle was then raised and the gear retracted and indicated normally.The gear handle was then moved down and the same result was seen as with the alternate extension; green nose gear; extinguished main gear and red light in the handle. Both main gear lights were tested and found to be functional. The aircraft was turned and headed back towards ZZZ. Several attempts were made throughout the flight to use higher speed; lower speed; side slip and elevated g to extend the gear. None were successful. Company base was called and notified of the condition. A low pass of Runway XX was executed for ground crew to get a look at the gear condition. All three gear were down and the doors were closed. Despite the fact that gear were down; the lock position of the main gear was unknown.After further discussion with base; it was decided to continue to execute the checklist procedures in an attempt to get a safe indication on all landing gear. After completion of all checklists the gear condition remained unchanged.Another fly past was executed at ZZZ and it was determined that the all three gear were down and the doors were now open. The state of the aircraft at this time was gear down; lock status unknown. Given the longer runway; barrier and more robust CFR (Crash Fire Rescue) it was decided to divert to ZZZ1. The transit time allowed for an additional checklist review; which showed; in the case of an odd gear configuration landing; a chute pull was recommended; hence one was planned.A low approach was executed for the Tower to verify the gear position; which they also verified were down. The landing was made from a long low straight in with ~800# fuel. Winds were light and variable. The plan was to use power to make as soft a touchdown as possible. Then; once all three gear were softly on the ground deploy the chute and come to a stop as gently as reasonable balancing the potential for gear not being locked down and the lack of anti-skid. The plan went as expected through chute deployment. Touch down was approximately 1500' down the runway. The normal step input decel from the chute was felt and light breaking was applied. Shortly thereafter and significant 'fishtailing' was felt similar to a wind gust on the chute or blown tire; which began pulling the aircraft right. Braking was increased on both wheels to moderate; with slightly more on the left to correct the aircraft motion and the aircraft came to stop on the right half of the runway resting slightly right wing down. The aircraft was then shut down. Post flight inspection shown that both tires had blown. The aircraft was otherwise undamaged and no injuries occurred.