Ok, but there is the question of why the landlord did not obtain a license. Was it because the property could not be inspected properly. Or did he just not know the law? And why can't he get a license now? Is it because the property needs to be vacant or, again, because an inspector can't gain access.
This article seems to pertain specifically to this situation:
So you want to be a landlord? Part 1 -- The Rental License - Avvo.com
McDaniel v. Baranowski, 419 Md. 560 (2011) is why the judge is refusing to hear an eviction case.
Since such licenses are specific to the county, we would also need to know the county to review the applicable law.
In any case, it seems that the tenants do know the law and probably went into this knowing they could take advantage and live rent free. There may be laws against taking advantage of the elderly, you may be able to prosecute the tenant for fraud (especially if they never paid rent) as they may have known there was no license and took advantage of it. Again a good local attorney might be able to figure a way around this.
Please keep us posted on how this turns out.