Should that be ok? I am transferring from Mh to Qf flight numbers but Mh to ek metal?

It's so confusing this interlining rule as there are no definite answers. Later this year I am flying ba Mia to lhr. I had to add on a separate flight ba lhr to fco with 4 hours connection time inbetween. I called to confirm I could get the bags checked all the way through and ba wouldn't confirm it,

It's hard to get a definite answer from the airlines

Also now we are flying Bne to sin on qf flight number but ek metal. Then separate ticket on Mh, will they through check the bags from bne?

Why can't they make a hard and fast rule. Something in black and white? Or maybe I'm asking too much