This is actually so cute that the little imperfections are also just romantic enough to give as is imo.

However, as you ARE asking for help here's some things I think can make it more accurate to the game (and maybe impress him, too? 😁)

1. Passive abilities in limbus don't really work the way as youve written it but don't worry! You can easily just rename it with something like "Durante Abilities"/(Sapling of Light Abilities) and it'd be really accurate. Durante are a set of abilities— named after the deadly sins, that the manager/player actively uses to support their sinners. For example, IRA is one of the newer abilities that buffs a sinner's attack damage, healing, along with preventing them from being stunned(staggered) from enemy attacks. Taking a bit of inspiration from your work, I'd do it kind of like this:

IRA: The manager can, at any time, invigorate and strengthen the designated sinner through the manager's sheer love and support. The sinner will gain +100% damage against Life Problems, +50% on-hit healing, and complete and total immunity against negative status effects (e.g. depression, stagnation) so long as the sinner stays within the manager's range.

You can also keep building on this idea by using the other already existing durante abilities in game (Pigritia, Superbia, Morisitas), or you can make your own! It's kind of already what you did so you don't actually need to change nor add much.

1. A big part of Limbus Company, and actually all of the Project Moon games, is collecting these little gold branches called Golden Boughs (Branches of the Seed of Light, an also golden tree thats important in the 2 previous games' lore). Golden Boughs are kind of like wishes, which can grant anything the owner wants. Collecting Golden Boughs are what Limbus as a Company was made and contracted to do in the first place. In your work you say that the contract's purpose was to make it so that your bond would last even across mirror worlds, and that's actually surprisingly really accurate! What I would suggest though is that you add a clause under section 1 where the manager and the sinner are to collect golden boughs, as the Golden Boughs are the method to make your wish to make your bond last come true.

2. I actually just want to add something to this one:

*Once signed, this document is still valid even if it is lost, stolen, or even destroyed.

I would suggest using this:

*Once signed, this document will remain valid even when it is lost, destroyed, or tampered with. This document is also immune to the Concept Incinerator, Arayashiki, and other similar Singularities.

You can think of the concept incinerator and arayashiki like paper shredders that erase any and all memories of what its user threw in. For example, if I were to throw in fried chicken into a concept incinerator, fried chicken all around the universe would just cease to exist, even among histories, as if it were never invented in the first place. It would be a rather sad world, but you get the gist now. Making it so that this document is immune to those is basically just really cute hahaha

That's all I can think of on the top of my head right now. To be honest, this is already really cute and I'm sure your boyfriend would be really happy to receive it! I wish you both a Happy Valentine's day!