Hey Felicia,
For "enterprise" (or multi-brand) solutions like this, I'll normally use different stylesheets for the Landing Pages and swap those out using a variable in the LP Editor so that you could basically toggle each brand on/off as needed (and load a different set of fonts, colors, etc). For the headers and footers (logos and copyright) I'll normally set those up as snippets so they're easy to change out. In terms of managing the different use-cases, it sounds like you're already on the right track there. The best thing I've found for this situation is to create a folder(s) of "Master Layouts" for each of your brands and then for each of your use-cases inside of that (eg. BrandA > Webinar Gated, Webinar Ungated ... | BrandB > Webinar Gated, Webinar Ungated ...) I'll set these up in Design Studio (or even better, a program template for "Webinars" in your Marketing Activities folder) so that they're available to clone from a pre-set starting point (pre-branded w/ the layout configured) in much the same way that you're doing with your quick-start examples.
The same goes for Email in terms of having a set of starter layouts to clone new pages from (aka quick-starts), but with Email, the rebranding isn't typically as flexible b/c you can't count on something like a stylesheet swap to handle all the changes you'd need to make from brand-to-brand. For Email, it might be best to choose one of your starting points (eg Webinar Invite) and get the layout/content all setup for any one of the brands on that one then clone it into a folder called "Starting Layouts" (or something like that) in Design Studio so you've got a clean folder with ONLY "Starting Layouts" that you can use into the future. Then clone that into the "BrandB" folder and rebrand it there, repeating for each other brand. Then do the same for each layout you'd want to use as a Starter (Invite, Confirm, FollowUp, Reminder ...) This'll help work around all the hunt-and-find from a previous campaign AND (my favorite part...) it'll also help to prevent the "Frankenstein" condition where you make a few one-off changes to the email and then those just kind of carry forward each time you clone that one. Using a "Starting Layout" and cloning it (rather than starting a new email using the template) will keep the starting point consistent and then you can safely go off-roading into one-off-wonderland on any cloned version of the Starter Layout without giving your future self a headache.
The reason you might be getting that error you mentioned is that there's a variable that's assigned somewhere in your modules that is not declared in the head -- either the <meta class="mktoString" id="VariableName" ...> variable got removed in the <head> or something about the ${VariableName} was changed in the code so that it doesn't match the id="" of the metatag any more.
In terms of editing Marketo templates outside of Marketo - anything is possible here, at the end of the day it's just code and you could use a basic text program (Notepad for example) if you had to. The trickier part is getting familiar with the Marketo syntax and understanding how to use that "on behalf of the end-user" (marketer) so that things like this are really easy to manage and suit your business needs. The piece that makes the templates difficult to edit outside of Marketo is that Marketo is translating the variables and tokens and other "Marketo stuff" like that when the code is rendered from within your instance but those same bits of code don't otherwise mean anything to anybody so without the translator in the middle (Marketo) it ends up looking like a hot mess. When I do want to try and troubleshoot or play with a new module layout/design outside of Marketo, sometimes it's helpful to preview the page/email and copy the rendered code out of the console tools in your browser's inspector b/c once the code is rendered it'll have all the variables and other Marketo magic removed. This creates a "clean" version of the code without the variables which makes it easy to tool around with, but then you cannot put the updated "clean" code back into Marketo b/c it won't have the variables -- so there's a bit of legwork to do once you know the solution in fitting it into the "Marketo-version" in your template or whatever that's got all the variables.
NOTE: In the email editor there is an "Edit Code" feature. I'd stay away from that all-together, it's a little deceiving and also easy to miss that any changes you make there will break your email away from the template it's using which could also be what's leading to the errors your seeing with the variable "freetextcolor".
Let me know if you've got any questions on the setup for multiple brands piece, and I agree with Phillip that it's not "easy" to edit Marketo code (inside or outside Marketo) and I think you'd probably end up way ahead by finding a team that's really well versed with Marketo (specifically) to produce something like this on your behalf.
Thanks,
Dave
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