If your provider uses the new 2021 Protocol, the equity release provisions will be agreed upfront, so you can get all the information before you commit. Details of the new IVA Protocol can be found here: 2021-iva-protocol-in-force-t91966
On the question of equity --- on paper as joint tenants you are each entitled to 50% of the equity. If you are tenants in common you are entitled to the shares relating to your ownership actually stated in the deeds. Again, as far as responsibility for the joint mortgages is concerned, unless respective percentages are part and parcel of the loan agreement ( unlikely), you are each jointly and severally liable --- so, if one party defaults, the other is fully responsible for the whole loan. Unless you have legally binding agreements regarding the split it will likely be assumed to be 50/50.
Regarding moving the existing mortgages elsewhere, during the IVA, this would be very unlikely, unless you can do this in your own name, as your husband will fail the credit checks and is also prohibited from taking any new credit without his IPs permission. It might be possible to downsize by keeping the existing mortgagor and securing it on the new property, but, again, as this would affect the equity available to the creditors, it would need the close involvement of your IP.
As the previous poster suggests, speak to a few companies --- you are likely to get a better service, in view of the potential complications, from a small to medium sized firm, rather than the factories, who are more 'off the peg'.
I am not qualified to give advice and can only state my opinions, based on my IVA experiences.