In my view the real reason for the outages is a policy decision on the part of IG to minimise liquidity risks. In my opinion, the most relevant part of that article is the one below
The fact IG is hiking up margin requirements all over the place and completely restricted trading in a few shares should tell you something. Given the lack of explanation from IG.COM about the cause for the outages, people are entirely justified in drawing their own conclusions (what you call conspiracy theories) based on the broader ongoing context. I'd rather hear direct from the horses mouth than you hunches and assumptions
You say you've been in system development for 40yrs. Perhaps this has made you overly technology centric? Your focus on your API integration is revealing. Most punters simply want to be able to log in to the platforms and trade at all opportune moments. IG seems to be copying the China playbook of shutting things down when it perceives a threat!
"In this situation, also A-Book brokers are not secure. There are many examples in the forex industry of STP brokers’ failures. Counterparty risk associated with liquidity provider can be crucial if we mitigate to them significant and concentrated exposures. In case chosen liquidity provider is not able to cover profits generated on hedge account, our business model will collapse like a house of cards.
Even if you decide to minimize the counterparty risk by connecting to the biggest liquidity providers or even prime of prime, in most cases such LPs are not flexible enough and cannot offer you negative balance protection. This can lead to a situation where your clients would generate huge negative balances on their accounts. Liquidity provider would ask to cover the negative balance on a hedge account, but the broker wouldn’t be able to reclaim money from traders. Such loss can be disastrous for your brokerage, especially as STP is a commission-based model."