You have got the overtime thing slightly wrong (or I might have read it wrong) ---- you are allowed 10% of your basic pay -- not 10% of the overtime then split the rest 50/50. Also check that your clause is the 10% 50/50 clause, as some are a straight 50 / 50 with no 10% allowance.
Most IP have traditionally dealt with overtime on a month by month basis but, lately, some are approaching it in novel and various ways, so you would need to check with your IP. When I did it I made the regular, automatic, IVA payment as usual and, every month, a separate manual payment of the overtime share.
On the expenses front, I had two co-op cashminder accounts and kept enough in Account One (where my salary went in) to cover the regular payments, DD.s etc. and transferred the rest into Account Two, for shopping, petrol etc. You are allowed to keep aside (save) small sums for things like repairs, haircuts, entertainment and whichever expenses you have been allowed which you wouldn't actually use monthly. On household shopping and the like, whatever you underspend one month you hang on to as you might well overspend the next month !
On the shared account: it is better that you run your own account and that you and your partner do not have any joint accounts as you will then be financially associated and your IVA could be discovered on credit searches on him and lower his creditworthiness (probably not significantly though).
For reviews, keep major receipts, in case they need to see them, but general stuff like shopping, haircuts, smoking etc. won't be needed (unless you have something like a special diet which costs more than normal). They tend to concentrate on income, rather than expenditure at review as expenses tend to stay largely the same, but income fluctuates more. They will want to see P60 and / or bank statements come the time, so keep stuff like this in a box file so it will all be easy to locate. Again different IPs will have different requirements.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014