There might be a way to add new waveforms to the Zadar, but the amount of "primitives" you have to work with (260) plus all of the complex transformation capabilities over those means that you probably won't ever have to do that. As for the ES-8 conflict issue, it shouldn't be a problem given that most DAWs these days allow you to specify which channels are being fed by/feeding which signals. You can probably set up a template that gives the Fireface and ES-8 clear directions as to who does what, and then use that as your main work template preference. I'd suggest going to Expert Sleepers' website (https://www.expert-sleepers.co.uk/) and looking over the Silent Way documentation as it relates to the ES-8 and using that with another USB audio interface. The other convenient thing, though, is that if you want to play live with only the modular, all you'd need is the DAW with Silent Way to handle your basic CV/gate functions with no need to pull the ADAT Lightpipes. And also, notice that the ES-8 has Lightpipe connections, so if there was a conflict, you could still fall back on those. But even moreso, the ES-8 has four inputs which the ES-3 doesn't have, allowing your modular to send control signals (such as clocking, triggers, etc) back into the DAW along with the capability of recording the modular's audio direct-to-digital.
No, the Erica cab doesn't have a fan. But what it does have is that amazing 140mm case depth plus excellent power specs, and the ability (if eventually needed) to add Erica's tube heater P/S addon for later tube modules, if you find some of those interesting. The Erica case is also vented, which helps to keep heat buildup issues (such as tuning drift) to a minimum.
Yep, modular is expensive. Nothing much you can do about that if certain functionalities that require specific modules are required. But while the build I put up looks complicated, ultimately, it's not. Everything's very straightforward as far as panel markings and controls, plus you have the ability to automate (via the ES-8) many of the Zadar functions, Akemie's algorithm selection, your mix levels, etc with the DAW/Silent Way able to track this via musical cues from the modular itself...yet there's still lots to tweak manually as well.