"combi-fare" and multi-centre booking question

Having recently started thinking about ways to burn some FC miles, I have a question about combi fares:
Would I be able to do M-G on the way out, and pay PE (probably the cheapest fare bucket) on the way back? My chat with an online advisor suggests that I can’t do this as a combi, as both sectors will be revenue fares. Am I right in thinking, therefore, that if both sectors are classed as revenue bookings, I won’t actually need to rely on it being a “combi fare” to “protect” me from paying the one way premium for the return sector?
An additional complication is that I’m potentially thinking about doing this as a “multi-centre”. The current thinking is that we would fly LHR – SFO (not sure how readily available those Gs will be) and then back from LAS (to LGW?). Not looking for this year anyway so there’s plenty of time, but it would be great to get a good understanding of how it all works. The other option (rather than the west coast road trip probably for 2 weeks) is a week-long visit to Chicago, where G availability seems pretty healthy. Appreciating that those are two completely different places to visit (but I’m putting them as alternatives because I only have enough miles to book one of them for now), any suggestions/recommendations/thoughts are very welcome!
Thank you
Would I be able to do M-G on the way out, and pay PE (probably the cheapest fare bucket) on the way back? My chat with an online advisor suggests that I can’t do this as a combi, as both sectors will be revenue fares. Am I right in thinking, therefore, that if both sectors are classed as revenue bookings, I won’t actually need to rely on it being a “combi fare” to “protect” me from paying the one way premium for the return sector?
An additional complication is that I’m potentially thinking about doing this as a “multi-centre”. The current thinking is that we would fly LHR – SFO (not sure how readily available those Gs will be) and then back from LAS (to LGW?). Not looking for this year anyway so there’s plenty of time, but it would be great to get a good understanding of how it all works. The other option (rather than the west coast road trip probably for 2 weeks) is a week-long visit to Chicago, where G availability seems pretty healthy. Appreciating that those are two completely different places to visit (but I’m putting them as alternatives because I only have enough miles to book one of them for now), any suggestions/recommendations/thoughts are very welcome!
Thank you
