The below is all personal opinion - not legal…
I always paint my walls with Silk, so that water based felt-pen marks can be wiped off.
I also do a manual written inventory backed up by photos and get it signed after walking through the key points with the tenant before handing over keys.
Personally, I dont think photos alone, taken prior to tenancy, will in itself count as evidence. In your situation, I would ask the tenant to cooperate in signing the inventory or risk “further action” although what that further action would entail depends on the content of the tenancy agreement and whether they are in breach having not signed the inventory.
Mark on the wall - difficult - if water based it should wash off. If they have kids there is going to be all sorts of dings and scrapes and muck by the end of the tenancy which if silk and/or glossed should wipe off or resist, but if Matt then will require painting. Feels like it is / is not wear and tear depending on scale and weather a sharpie was used as opposed to water based - don’t quote me I’m not an expert.
The following narrative precedes my Inventories and is plagiarised from the local council for a council tenant agreement. This might help?
What is ‘fair wear and tear’?
To avoid confusion for both landlord and tenant, here is guidance as to what we feel constitutes fair wear and tear.
Any claim for damages must be made in writing within 28 days of the tenancy ending. All claims should be accompanied by evidence of the claim.
Decorations have an accepted life expectancy. However there may be circumstances where excessive wear and tear requires compensation to make good, e.g. numerous nail or picture pinholes, torn wallpaper, gouges in walls/woodwork etc.
To avoid accusations of betterment against us, the landlord should not end up, either financially or materially, in a better position than we were at the commencement of the tenancy, allowing for fair wear and tear. The allocation or apportionment of any costs, charges or compensation for damage must take into account all the factors relating to
(a ) Fair wear and tear
(b ) The most appropriate remedy and,
(c ) That the landlord should not end up either financially or materially in a better position then he/she was at the commencement of the tenancy or as he/she would expect to be at the end of the tenancy having considered (a) and exercised (b).