ETFs and Diversification
Since ETFs are already diversified by virtue, and should you pick an ETF that is built from several other ETFs, then this strategy could make sense. MoneySense has an in-depth article in response to this subject by one of their readers.
The critical caveat with one single ETF is to chose well which one you buy, and what the fund is made up of.
Mike Philbrick on ETFs and Diversification
Could be possible, with a good investment advisor. The recommendation here is the HRA-T ETF.