Hello, and welcome. I'm a licensed attorney and happy to assist.
You definitely have a lousy landlord, but unfortunately the laws of commercial leases are not the same as those of residential leases, which afford tenants a lot more rights. Commercial leases and laws favor the landlord in the sense that the landlord has virtually no responsibilities to the tenant and answers pretty much only to the City Building Department, etc.
1. Even if the structure was deemed illegal when you signed your lease, the lease is not automatically void in a commercial tenancy. This would be different in a residential tenancy.
2. Since commercial landlords aren't held to the same standard as residential landlords, there may be no grounds to sue for damages under the circumstances. You could sue for damages only if you paid rent and did not get what you bargained for (for instance, if the landlord knew (a) you needed to obtain a permit in order to conduct business in his building; (b) that his building didn't comply; and (c) the noncompliance would necessarily result in denial of the permit). In such a case you could get your money back and perhaps damages under the doctrine of failure of consideration. Other than that, there would be no way around the "as is" provision. Unjust enrichment is applicable only when there was not an actual contract, so it not of use here.
3. See above. While the lease would not automatically be void, you would have grounds for getting out of your lease if the landlord knew you could not actually do business in the building.
4. The personal guarantee should have no effect on whether or not you can sue the landlord.
I hope this helps. If you need additional information or clarification, just let me know and I'll continue to provide assistance. If I have addressed your issue and/or pointed you in a positive direction, please let me know that, as well. Also, please remember to leave a positive rating when prompted, as that is the only way attorneys on this site are credited for the help we provide.