Do developing country equities have a higher return and/or lower risk
than emerging market equities?
Generally in finance you get payed more for taking risk. Riskier stocks over the long run return more than less risky bonds, for instance. Developing market equity is expected to give less return over the long run as it is generally less risky than emerging market equity. One way to see that is the amount you pay for one rupee/lira/dollar/euro worth of company earnings is fewer rupees/lira and more dollars/euros.
when measured in the emerging market's currency?
This makes this question interesting. Risky emerging currencies like the rupee tend to devalue over time against less risky currencies euro/dollars/yen like where most international investment ends up, but the results are rather wild. Think how badly Brazil has done recently and how relatively well the rupee has been doing. This adds to the returns (roughly based on interest rates) of foreign stocks from the point of view of a emerging market investor on average but has really wild variations.
Do you have data for this over a long timeframe (decades), ideally for multiple countries?
Not really, unfortunately. Good data for emerging markets is a fairly new phenomenon and even where it does exist decades ago it would have been very hard to invest like we can now so it likely is not comparable.
Does foreign equity pay more or less when measured in rupees (or other
emerging market currency)?
Probably less on average (theoretically and empirically) all things included though the evidence is not strong, but there is a massive amount of risk in a portfolio that is 85% in a single emerging market currency. Think about if you were a Brazilian and needed to retire now and 85% of your portfolio was in the Real. International goods like gas would be really expensive and your local currency portfolio would seem paltry right now.
If you want to bet on emerging markets in the long run I would suggest that you at least spread the risk over many emerging markets and add a good chunk developed to the mix.
As for investing goals, it's just to maximize my return in INR, or maximize my risk-adjusted return.
That is up to you, but the goal I generally recommend is making sure you are comfortable in retirement. This usually involves looking for returns are high in the long run, but not having a ton of risk in a single currency or a single market. There are reasons to believe a little bias toward your homeland is good as fees tend to be lower on local investments and local investments tend to track closer to your retirement costs, but too much can be very dangerous even for countries with stronger currencies, say Greece.