@op; (this is at your risk, and you really need to speak with your SE first)
1. make the length of the 'offcut' of UB fill the space between two rafters, and stabilize it with 100x100 angle brackets screwed to the sides of the
rafters and tight against the web - you'd need 4 brackets, 2 each side. --- blue on sketch.
2. no prob. cutting the outer half of the flange off.
3. the suitability for shear strength of the chamfered beam will depend on the load the beam is carrying, and how much vertical length of web is left at 1/3 the distance in from the inner lip of the 'spreader' beam ---- red line on sketch. This length will probably be approx. 30-40mm - your SE would be able to check as he will know the figures.
4. inspector might expect a padstone - not necessarily required structurally as it depends on the load from the beam and the strength of the brickwork,
but again the SE would know this.
5. Leofrics' point above is a valid one, which is that the load from the floor joists seems to be taken on the rafters - are they suitable for the increased load, and has anyone considered the type of fixings needed?
Bear in mind that the overriding issue in this is not the strengths of the individual parts - these can be quantified and proved - but overall stability of the detail and you have to be sure you/the builder know what you're doing. You are trying to make the best of a bad job, but you are where you are - as they say.
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