This is one of these terms that varies regionally. In the US it's 'takeout', in Scotland it's 'carryout', in other places it's 'takeaway'. There may well be other, equally valid, terms in other English-speaking countries. Things that 'sound bad' to one English speaker are perfectly normal to others.
As for the countable/uncountable issue, both can work and I think your reasoning is spot on.
The countable option seems to be preferred in the British/Australian/NZ convention (as confirmed by Gary and Hilary). 'A takeaway' suggests a single order of food from one outlet. If you get several orders of food from different outlets, you could say '[some] takeaways' (as Gary suggests).
Meanwhile, the American convention seems to be uncountable in all cases. This is confirmed by Chris (who didn't seem worried about the term 'takeaway' but still applied uncountable grammar) and DavidK (who rejected 'takeaway' and was particularly unhappy with the double wrongness of an unfamiliar term and seemingly incorrect grammar).
Can you combine the conventions? I don't see why not. As is always (and increasingly) the case , the world at large is happier with US conventions than Americans are with British ones. Although BrE speakers tend to use the countable form, it wouldn't be wrong to use the uncountable : 'I got some takeaway' sounds fine, and it's a nice compromise to cover a situation where you multiple orders from several outlets.
Forget your conspiracy theories. When it comes to countable and uncountables, there's probably more variety than you'd think. Providing the internal grammar of the sentence works ( i.e. correct singular/plural agreement of modifiers and verbs), the possibilities are actually quite flexible. Let that be your 'takeaway' from this experience?