Thanks once again to all who offered advice. It was all used and appreciated. The search for tea was successful. The search for Celtic Connections concerts of great worth was successful. The search for daytime fun in the form of excellent museums was successful. The search for sessions was not quite so successful. Like anywhere, these things are frequently flexible and/or changing.
I did go to the venues below, and either enjoyed the session - most particularly, GREAT fun at Uisge Beatha, Woodlands Road - or got information from the barman or woman about the current session status. I'll just paste below what I was told.

Scotia Bar, 112 Stockwell St, corner Howard - unscheduled sessions
Lismore Pub, 206 Dunbarton Rd, near Gardner St - Tues & Thurs
sessions, 9:30 on
Victoria Bar, Bridgegate, corner Stockwell - Fri night tunes session
Babbity Bowster, 16 Blackfriars St, near corner High St - Sat tunes
session, 4:30 on
Uisge Beatha, 232 Woodlands Rd, near Willowbank - Thurs tunes session,
Sun mixed session, both roughly 8-midnight