You raise valid points. They're all things to consider when setting this thing up. Nothing so far convinces me that the idea is sure to fail. A lot of the difficulties you raise would've applied to the first real-life banking systems and yet they found ways to make it work.
Infiltrators: they tend to charge a lot for their deeds, so bunch of gold down the drain right there. Plus: I've been the target of many infil attacks, never really bothered me all that much. Wounded for a few days? Oh well, I'll just focus on my other characters a little more in the coming days. Usually, though, the attacks just fail, I begin patrolling the streets and the infil is caught soon after. It is no secret that I think very poorly of the infiltrator subclass. Even if you had infils continuously stab me, which I doubt you could, I'd just scoff it off: sooner than later they'll get caught, maybe tortured, banned, and next time their head will roll.
It's a managerial decision how far you're willing to go to collect. You have to weigh the cost of enforcement against the cost of doing nothing. Some small debts might just get written off, but the assumption is that you'd only give loans when there's a reasonable expectation that you'll be able to collect. Even if you don't get back every penny, the use of retaliation would at least be a deterrent.
Government members, priests, armies? Really? What do they care for your guild? Even if you had powerful friends in on it, they'd probably never get away with using the realm's resources for your guild's aims.
They'd have to have some sort of stake in the system. They might also be borrowers. You can imagine a situation where a realm needs to build up its military very quickly so they may want to borrow a large amount of gold, and as a banker you could attach certain conditions to the loan.
Closing guild houses: This also actually costs gold too. It'd also mean lowering the maximum full member count and reducing places where people can join the guild and interact with it.
There may be a one-time cost associated with closing a guild house, but it would be worth the expenditure if that guild house is causing your bank to lose money.
All of your solutions to making people pay is to spend a ton of money, basically, a ton more than what they owe you. By all means, try if you wish, and if you'll have fun trying to make it work, but you could basically do your whole scheme even more easily by keeping track of everything you want to do on a spreadsheet on your computer (and/or on the wiki), instead of using an IG guild. Suddenly: no upkeep, no taxes.
Good suggestion.
