If she were to resign her seat in an ego-huff, Labour would almost certainly hold it and hold it easily (and quite possibly with a larger % majority than she got in 2005; by that point her credibility had gone and she had become an extremely unpopular M.P). All four wards in Ladywood voted Labour this May; Soho and Ladywood are Labour strongholds ('though there are some mild concerns about gentrification in the latter), while the LibDem threat in Nechells seems dead now (big swing to Labour there) and Labour regained Aston (and by fair means this time) by a larger than expected margin as well.
I'm not so sure... it's a by-election and the Labour Party is in a very weak position at the moment, I'd expect the LibDems to be able to take full advantage of that, remeber that in Bromley the local LibDems had just gotten hammered by the Tories in the local elections and yet they nearly won the seat and the Tories where in a much stronger national position than Labour our today... in the end, if there was a by-election (which i doubt) I'd be surprised if Labour held it.
