It's actually quite lucrative. If you think about it, one claim with one employee in one company is likely to snowball through word of mouth.
I had this very thing last year with a relocation of a team. I priced on % refunded and it was a handsome fee for about a days work.
Your client company is making Exempt supplies of rent so, yes, an OTT is required to recover any input VAT and this should be done before any supplies are made.
I take it there was not a tenant at date of sale? If there was ToGC may have been in point.
Everything you list are just tools and reports with which you may provide advice. They aren't an advisory service.
An advisory service needs some advice and interpretation of those reports and, to my mind, need to be considered in view of the business' objectives which may range from something as simple as "make profit" to a more specific objective of "increase sales of product x in region y".
There isn't a formula for it. You need to engage brain.
I consider it a privilege to be able to help friends and family. If I cover staff costs, that's enough for me (although Mum n Dad are FOC).
I'd sooner know they were in safe hands than elsewhere.
Like hen's teeth
"A new client came in to see me today, they brought in a large cardboard box, they put their hand inside and pulled out a ..."
decent Aweb thread.
The answer to most of your questions is - you're the boss. You decide.
Find a brute force cracker and let it run. May take a while. I don't think excel files lock on X attempts so shouldn't be a problem.