Unfortunately, with these things, it's so often a case of trying this and trying that. My personal policy when dealing with anything that involves potential crop issues or poop problems is to go back to basics and work from there. Sweetcorn is not only high in sugar and will therefore feed any bacteria (good or bad) but, just like in humans, is quite difficult to properly digest and often comes out not too dis-similar to how it went in (sorry - gross!) If there are crop issues, then you need to make things easy on it for a while, with soft, easily passed foods. Whenever I've had birds with crop issues, I have fed pellets if they take them, egg (chopped or scrambled) with added garlic and natural yoghurt and live maggots from the fishing shop. The maggots remain alive for some time and will eat any rotten matter in the crop and I believe the agitation caused by their movement also help things to move along. The egg provides nutrition, the garlic is great in a whole bunch of ways, but will also help with circulation if that comb of hers has frostnip/bite or even just general circulatory issues and the yoghurt helps provide good bacteria both in the crop and the gut. I, personally, have never used, or needed to use, any 'medicaments' for crop issues but as long as they are used sparingly, I can't see them causing problems if the crop is, in fact, just fine.
Be aware that alka seltzer will fizz quite profusely and you don't want anything coming up and being aspirated so go easy xx
If you can get Nutri Drops, they are without a shadow of a doubt, my absolute 'go-to' when dealing with ANY illness. They really can give that much needed boost when everything else seems to be failing or taking too long to give results. The fact that they go straight to the bloodstream rather than relying on digestion is a huge bonus when the digestion system is an area for concern.