You need an array of enforcement tools. If you involve the right people with enough power and influence, then I think you can find ways to effectively deal with any of the problems being mentioned. For example, you can hire infiltrators to go after delinquent account holders, set bounties, use judges, bankers, rulers, priests, etc. In extreme cases, you might resort to military force. Lords that try to exact unfair taxes may get their guild house closed down, or they may wake to find an assassin in their bed chamber, or they might have their lordship taken away by the duke. Perhaps guild members could be asked/told not to sell food or provide any assistance to lords that have been blacklisted by the bank. The key is to have a multitude of tools at your disposal and get the right people on your side.
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.
Bounties: Same principle as above, a new huge money dump. Plus, bounties rarely really attract infiltrator actions that wouldn't have occured anyways. I've seen a bunch of people, myself included, with high bounties for very large periods of time. When my characters eventually get attacked, it's usually by a realm I'm at war with by someone who would have tried it anyways.
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.
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.
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.
Setting up a bank that isn't meant to just fund realm efforts is a complicated task unlikely to work by itself, but if it is to work, guilds are most definately not a good way to go about.
Guilds' primary use is for coordinating multi-realm military activities. Their secondary use is RP. Guilds with other activities rarely succeed in their aims.
