You are talking mostly to Americans here. They don't know nor care about the availability and possible tax risks of owning their preferred US funds for non-US investors, so some of their replies might be kind of useless for you.
Same can be said for their refusal to diversify. For many of them 100% home bias is perfectly acceptable, they believe they can do without "third world businesses" like Samsung or "International" like 100% of Europe for example. They also preach indexing their home market, but strongly advise to time the market or even pick stocks for 50% of the global market.
You decide if such advice is useful to you.
With the MSCI World and EM you are invested in 80% or so of the global market, except some mid and small caps. Which is fine and certainly better than not investing at all, and all a new investor needs to get their feet wet. You can add the missing "Vanuatu Island Total Stock Market", "Himalayan Economy above 4000 m"and "Vietnamese Water Buffalo Industry" and whatever other exotic ETFs you consider necessary at a later date, once your invested capital exceeds a high 6 figure sum.
Both the Comstage and Amundi ETFs are equivalent to each other, as far as I know they are absolutely comparable and there's no real difference between them. If you already own Comstage then it can't hurt to choose another ETF provider.
Regarding the synthetic replication, as far as I know there aren't any ETFs for MSCI EM that are fully replicating. Optimised sampling ETFs exist, but they cost more.
If you ever decide that you prefer other ETFs you can start buying them in the future and use/sell l the unwanted ETFs for rebalancing purposes over time. Diversification over several ETF providers can't hurt either.