air in the hydraulic tappets and engine damage

A collapsed lifter will cause damage to the top end of the engine and valvetrain and you might hear it clattering and ticking as it runs. If you have damage on the bottom end (crankshaft connecting rods, camshaft), that would be an indication of oil starvation, and low oil pressure would be an indication of an impending issue.

I'm skeptical that this severity of engine damage would happen due to one or more collapsed lifters.  And while not desirable, even an imperfect oil system purge will likely purge itself in the first few seconds of operation. I think the main clue here is that the oil pressure was immediately lower after the hose change. A lifter problem would not have caused that. The indications are that something went wrong during the hose replacement that restricted or limited the oil supply. I have a couple of thoughts:

1. Is it possible the mechanic blocked the oil tank vent line as part of the purging procedure and accidently left it that way?  A blocked vent line may cause oil not to return to the tank while the engine is operating. You may not notice this during your preflight if you remove the oil cap to burp the engine. 

2. Is there an oil thermostat on this engine?

I would tend to agree that such damage could not be caused by air in the lifters alone. Also agree that a blocked vent line or collapsed hose on the suction side, could. (Oil pressure not rising or falling with increasing rpm is a sign of a collapsing hose.)

Another possibility: a leaking oil hose fitting, especially ones between the tank and oil pump inlet. The lines are under vacuum, not pressure, so could suck in air while running and not leak much, if any, oil out. This would also account for the lower observed oil pressure.

thanks to everybody for answers and advice. 

i forgot to mention that oil press goes very low 1-1,2 bar) ONLY when engine is warm. otherwise it is lower than usual but within spec (around 2,5-2,2 bar).

Usually in engines this can indicate a bearing damage. (see attachment)

Rotax techs do not check the oil hose under suction but only with positive pressure. we'll do this check.

oil tank vent is clear.