To be granted a Reservist Award you need proof of earnings. Most self-employed bods will not of course have P60s etc., so taking along the last few Tax Returns seemed to suffice. Be advised that this may have altered in the last couple of years so you may want to contact RTMC directly and ask their G1.
Yes, you can claim for any difference between your service pay and earnings from your business up to a cap.
You are also entitled to claim as an an employer, if you needed to recruit someone to replace your role in your business while you are mobilised for example. This is also subject to a cap. When you return from mobilisation, if skills you need to be able to return to your role have lapsed as a result of your mobilisation, you may be able to claim for the cost of re-training.
Generally speaking you're swapping one job for another so all the allowances/benefits available to an employer are not available to you. Generally not an issue unless you are hiring someone to cover for all or part of your job whilst your away. At one time there were elements of re-training grants that were available ask someone other than your PSI to point you at the right part of Glasgow. That aside it's all pretty straight forward.
You are not the first self employed person to go through the system, so whilst you may feel rather special the system might not regard you in the same light.
Your accountant can produce a letter telling them what you earn and what benifits you take etc. You then just need to take tax info and anything relating to tax code etc a proof in the defence of your claim.
Becareful with what you claim though and them may check so simply getting someone to write that you earnt £100000 in the last year will look odd
things like petrol and cars if you have that as part of you wage will be taken off too as thats a perk, one guy got very upset during the last visit to chilwell as his £10000 car allowance was written off so he got paid less
The simple answer is its all down to you to prove what you earn, if its lower than your rank pay you'll get paid the pay for your rank, if it higher then you'll get the award.