Open Table Formats (OTFs) enable interoperability across systems by using object stores as a shared storage layer. They support OTF HTAP, in which complex queries execute concurrently with updates on the object store. However, existing OTFs provide transactional guarantees only for single-table transactions. Supporting transactions that access multiple tables typically requires external coordination services, which compromise interoperability and deployment independence. In this paper, we show that this trade-off is unnecessary. We present OTF-native mechanisms for multi-table atomicity, consistency, and isolation that rely solely on object-store primitives. We realize them in LakeVilla, a prototype that provides full ACID transactions while remaining compatible with existing OTF deployments. Our evaluation combines formal verification, extensive benchmarks, and an interoperability case study with Trino, showing that LakeVilla delivers strong transactional guarantees with low overhead, scalable concurrency, and compatibility with existing analytics engines.