Are unorthodox, unmentioned frequencies possible? Use of trigger conditions to change the allowed periodicity is possible, though I might want to make a device that requires less levels: a device that works once a month? Once a year? I can see how it could unintentionally make single-use devices too cheap, so that's an issue to consider. What would be your input and advice?
Making such "once-in-a-while" devices is in principle possible, but, by the Rules As Written, it does not save any levels. As you correctly point out, for game balance reasons the rulebook wants to avoid making single-charge devices easier by making them usable once-per-century. You could avoid that issue saying that the "low-frequency" devices cannot have charges (like Constant effect devices), but even then, any low-frequency bonus should be worth less than the bonus derived from making a device single-charge (-5). Or perhaps you could say that you can get much larger level reductions for really long inter-use intervals ... if one has to wait for that long after the item is created before first using it. I'd have no issues with a wand usable once-a-century getting a -20 level reduction, for example, if one had to wait for a century after its creation before using it for the first time.
Speaking of charges… What is the reason for the charge quantities that elevate the level requirement? Wouldn't it be prudent to just enchant the item without those limitations? Or integrate a counting system if you find this absolutely necessary? Is it possible to instill even more charges?
It's not possible to "expand" the charge limit once the item is created, only to "top up" used charges. In general you are right, creating charged items with many charges (24+) is inefficient, but it does guarantee that the item will eventually stop working if it falls in the wrong hands - for example, the Order forbids selling enchanted devices to mundanes unless they will eventually exhaust their power.
Speaking of instilling… How does Constant Use interact with durations? Which duration shall I choose? To reword these a bit: does Constant Use prolong the durations or require any?
Constant effectively "prolongs" the duration; you should thus use D:Mom.
I only have the 4th Edition Core right now, and I'm unaware of any previous forum threads on those matters. Any links, quotes and general ideas would be greatly appreciated!
This forum came into existence with 5th edition (20 years ago!) as a friendly venue for new players unfamiliar with previous ones. Thus, discussions almost exclusively focus on the 5th edition rules and setting, which are widely considered the best, at least overall (4th edition, in contrast, is probably the weakest edition mechanically). There are a number of resources for previous editions on the web though. The best point to start in my opinion is Project Redcap.