Thanks for all the input. Seems clearer to me now. Basically on sep tickets with different full service airlines, if we have no luggage and onward BP it'll be fine, but onward BP with checked luggage will mean having to go through immigration, unless first airline interlines your luggage.

Transfer counters good for printing BPs based on printed check-in confirmations/BPs, but could be more problematic when BP is on the phone.

Hopefully I can report back my experience from April.