#debasement

It’s possible, but I think it’s unlikely for a couple reasons.

First, unlike a lot of Littlefinger’s financial manipulations, debasement of a currency is pretty easy to spot, either through the weighing or assaying of coins. And, unlike a lot of his scheming with regards to the Starks and Lannisters, Littlefinger is very careful to cover his financial tracks - all the way up to murder in the case of the false Antler Men.

Second, some of that evidence doesn’t quite hold up. The whole point of Lady Olenna’s Gardener gold is that they’re clearly (noticeably smaller and thinner) worth less than regular dragons. If there was widespread debasement going on, you wouldn’t see artisans complaining about being paid in Gardener gold. Moreover, it’s not surprising that there are Westerosi coins floating around in the Free Cities - it’s a nearby major trading partner, and one that probably has an unfavorable balance of trade, so you’d expect to find more Westerosi coins in Essos than vice versa.

Third, given that there’s pretty clear inflation going on because of the War of Five Kings - pillaging and blockades reducing supply, changing liquidity preferences (”can’t eat gold”) amid massive uncertainty about the future, the disruption of economic markets more generally - it’s hard to tell whether any evidence of inflation was caused by debasement or other factors. So you could be right, you could be wrong, it’s pretty hard to tell.

If you’re interested in real-world medieval debasement, I’d recommend Christine Desan’s Making Money (as I so often do). To answer your questions: 

debasement was advantageous when a monarch needed extra cash or was in debt, or more broadly when there was high levels of deflation or high levels of indebtedness holding back economic growth. However, it was less advantageous if the reverse applied, because it can induce inflation, reduce people’s real purchasing power and real wealth, and potentially cause capital flight and problems with international trade if foreign merchants started insisting on being paid in gold. Think of it a bit like inflation and deflation in modern economic policy - whether they’re good or bad depends a lot on the circumstances.