I am no expert on the nuances of welfare, so I defer to you and others on that.
As to the bond markets, they want ‘fiscal stability’ more than anything, and if the Old Lady cuts 25 bps, which they will, whether it is next week (unlikely) or in December, that will also act to ease yields. If Reeves implements more aggressive tightening (read higher tax rises etc), political fallout aside, that would also be a positive for the bond markets.
Addressing the probability of the need for an IMF bailout, I find the notion fanciful - this is not a sovereign crisis (at the moment), as in the UK can access the bond markets, and is able to meet its financial commitments. The 1976 bailout was fundamentally a crisis in sterling, coupled with rampant inflation.
Whilst I would like to see yields spike to put Reeves and this government under pressure, the bond markets and our personal willingness to bear more and more tax, are uncorrelated (if one excludes the political risk from Reeves piling on more taxes, whilst unable to cut expenditure.)