I appreciate your concern on my behalf! However my relationship with local charity shops is informal & based on trust. I'm not obliged to address any watch or clock that they consider needs attention. What I get out of it is a series of opportunities to learn & practice - assisted by WRT colleagues, my books & YouTube videos - & the satisfaction of the occasional 'notable' (to me) success & of being of some use to good causes.

Should I attempt this with the spring on the balance wheel, or should I take it off?

You must put the balance in movement and see how the roller is situated against the pallet fork. The dart (guard pin) must point against the middle of guard roller. The impulse jewel is not needed at this point. The balance should rotate free and the dart must have normal clearance with the guard roller. This means the pallet fork will move just a little and will not unlock and will return to the banking pins when pressed towards the center.

I'd had a similar thought to @RichardHarris123 using something like this, but ideally the same ID as the case.

Thanks for the warning. I had in fact picked up the point about the risk to tolerances in the thread referenced above, so in an attempt to minimise the problem I made a cylinder to effectively 'bush' the RT tube rather than 'shimming' the staff on one or 2 sides. The table now seems secure although the base of the table is not *quite* in contact with the hub. I could possibly press/tap it down further but am chary about that given what I've been told about the fragility of roller tables. I did in fact conclude that the 90° orientation would put the jewel in the right position wrt the pallet fork etc so your comment is reassuring. I'm now left with another 'first', viz replacing & securing the jewel which popped out as I removed the large, untidy splodge of shellac underneath the table (this, incidentally, was preventing the table base sitting right on the wheel hub).