I've just tried to get an answer from the BA service line but the agent turned to be really incompetent and couldn't answer my questions. So, giving it a try here.

I'm planning to travel to South America (Chile) on a mix of IB/LA/BA flights in March, perhaps the ticket will be issued by Iberia because they come first. On my way back for the reason of getting a competitive Premium Eco fare, I plan to fly from Santiago to London via Buenos-Aires (LATAM + BA). However, I'd like to continue my journey from Heathrow elsewhere (LHR-DME) on an award BA ticket issued by AA.

1. Is there any possibility to check the bag through to DME directly from Santiago given that there are BA flights on two separate tickets? Given how much headache it is to clear the customs at LHR then re-check the bag and get back on the next flight, I'm wondering if there are any options to simplify the procedure.
2. I'll be traveling with my colleague who has Russian passport and no British visa, so for him it's not an option to go outside in Heathrow. If he travels without checked bags, will he be able to re-check on the BA flight to Moscow on a BA transfer counter before clearing the customs? Will the existence of a separate LHR-DME ticket be a sufficient argument for him to be admitted on the airplane from EZE to LHR?

Thanks!