Welcome to expat.com @miksandstrum!
It's the land that causes the company requirement, so I suppose it makes sense that it would be possible (and legal) for you to buy the house personally, and the land in your existing company. Sounds like a bit of a faff to me, and I'm not sure how advantageous it would be. This is the first time I've heard of it, but perhaps it's more common than I realize. To find out the exact pros/cons, you probably need to consult a lawyer or other real estate expert. I'd personally do exactly what you did before (i.e. use the company to hold the house and land), unless your realtor has some strong arguments for doing something different.
As discussed previously in the topic, the lower declaration is officially illegal but very commonplace. It lowers the seller's capital gains and hence tax payment, and this is the main reason for its popularity. (However, this is not the only reason: it also lowers the transaction fees and property taxes, both of which are down to you, the buyer.) If you can say "no" and not lose your seller, or you can easily find another property, then it isn't a difficult decision. If it's an urgent purchase or you can't afford to lose the property, then I suppose you might be more tempted to agree. As it's about lowering his tax payment, you'll find that, mostly likely, if you do refuse, the seller will want a higher selling price (to get the same net amount).
As to the value on the notary deed being used for insurance purposes, that sounds incorrect. Someone might have bought a house 20 years ago for 1,000 leva! When I purchased some house insurance last year, they asked me to specify values for contents and replacement. The cost varied based on the total I asked for. So I am confident they pay this, rather than looking at my deed.
Compulsory purchase might be a different story, but it still sounds pretty harsh if they can buy based on the price 20 years ago. In the UK (and most countries) the compensation is based on the open market value. But perhaps there is some legal relic of communist times that hasn't been updated. You sound pretty certain, but I'd be surprised if this were still the case. :-)
A quick search online turns this up (in relation to Bulgaria and other Eastern European former communist countries, or "transitional economies"):
"After 1990 the newly elected democratic governments changed the constitutions to permit and protect private ownership of land. However, compulsory purchase is essential in a market economy to deal with certain aspects of market failure. In spite of their commitment to the inviolability of private property, the transitional economies have had to develop compulsory purchase procedures and means of assessing compensation."
So... overall... illegal, and not recommended. But if you like your new house, it will probably be easier to just go with the flow. And, in my opinion, unlikely to result in any negative repercussions (the notary police knocking on your door, arguments about the insurance value or compulsory purchase value).