@Jessa85
One thing to be careful of is 'adding' or 'subtracting' money that's in your name/joint accounts. This is where Grandad giving you the money to pay the legal fees may present a problem. Best bet is to consult the solicitor. Perhaps Grandad will need to frame this as a gift or a loan. IANAL, but I don't see your grandad paying your legal fees would present a problem if he's not directly handing you money. I'm sure relatives (parents, etc) pay legal bills all the time for those who can't afford them. I'd ask the solicitor if you should have them send the bill directly to Grandad though. Cut out the middle man (you).
My (high earner) son is going through a nasty divorce and was cautioned that, up to the agreement on finances, anything he receives can be considered 'marital funds' and anything 'pricy' he buys can be considered as a joint asset for division of property or she can potentially claim half the purchase price in the settlement. For example, she took their car. His attorney told him not to buy a new one until after the settlement is signed because she can potentially demand the car or 1/2 the value in money in the settlement, even though she already has their car. He was cautioned also against even buying us expensive Xmas gifts lest she claim that he's trying to 'deprive' her of marital money. So just be wary of money going into or out of accounts in your name. But the same goes for monitoring HIS spending. Keep an eye on things and make copies of any unusual or large purchases or expenditures.
Our STBXDiL cheated on our son then begged forgiveness. We loved her and never thought of her as nasty or vindictive. But once she realized he wouldn't take her back, all hell has broken loose. I guess what I'm saying is do not trust or underestimate your STBXH. Either in hiding money or in demanding money from you.
As far as pension/assets, he will be required (under penalty of perjury) to complete a financial statement. At that point you can review it with your solicitor to determine if he's disclosed ALL pensions and/or investments/assets. This is why you may want to quietly start digging for & compiling information now. Former employers, financial institutions, investment firms etc. Copies of his tax returns. All may be leads to assets. Forensic accountants are very expensive, the more information you can provide them, the less they will cost you.
Play your cards close to your vest for now and IMHO do not tell him about the divorce until you have the paperwork ready to serve. To tell him now will give him time to start hiding things or to 'get in there first' with filing. The one who files as a rule is the one who controls the time line.
This will be a marathon, not a sprint. Prepare accordingly. And best wishes!!