Case 1, correct. If you can't take a tail-log backup, then you can only restore to the last log backup you have.
Case 2, Full backups DO NOT and NEVER have broken the log chain. Ever. Restore Monday's full then all log backups up to the last one you have.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability