Idea...Have mom list her home, sell it, put the proceeds in the bank in a joint account with you on account.The wait a few months, then purchase another property as an Investment Property with 20% down.Move mom into it, remove yourself from her account, then do a Lease Purchase, (record the lease) structuring it with you giving a credit to pay closing costs and after 12 months of having her making payments to you,  have her refi into her name.  

It's important to determine what you are looking for in your ROI numbers especially if you plan on Joint Venturing these out.

I started by doing joint ventures with homeowners, contractors, investors, etc.

I think the legal term is jointly and severally, but I'm no lawyer, this is not legal advice, and I could be totally wrong about the terminology.That way you go after both of them until you get paid, or whoever has the money.And I'd NEVER let someone move in who is not on the lease. 

If the rent is $1200 pm and two people apply they should both be earning at least $1800 pm, £3,600 joint monthly income.

I have looked into adding my name to the deed as joint tenants, or tenants in common but my biggest concern is that if anything were to go wrong (e.g. while renting it out), my dad’s name would still be on the deed and therefore still partially liable (I would like to assume full legal liability).

Here's where it gets odd.Jack wants me to be the sole borrower on the property, on our joint project.Jack wants me to not tell the bank about my source of rehab funds because it'll "complicate things with the bank"The bank has capped my borrowing limit at $250,000, which is above the necessary $295,000 to do the deal.As a workaround, Jack will be the sole provider of rehab funds (personal funds, cash)Jack wants me to give him one of my personal checks so he can write future offers.Is this a strategic way to expedite the rehab-lending process?

looks like you no longer have this issue.But in the future if it happens again what's been mentioned so far - screen the new tenant, fill out application form, collect app fee, and have all existing tenant sign.IN ADDITION, make sure the existing tenants and new tenant understand, and include into the addendum if necessary, that all claims, damages and deposits are shared by all tenants from this day forward.That means if you see a broken window and need to charge them for repairs tenant C can't say oh it's broken before I moved in.Or if tenant B moves out and six months later you charge a deduction in the security deposit on a hole in the wall behind the couch he says the hole wasn't there when he moved out so it's not coming out of his deposit.At the end of the lease when you refund the deposit it will be a check to "A B and C" jointly regardless of who might be there or not at the end of the lease. 

My wife and I have completed a 1031 exchange of a property we owned in joint title. 

 We use buildium property managment software which allows tenants to set up online rent payment via ACH.