I very much like the first two ideas. The third I'm not sure on mainly as I don't have much family gold right now to actually spend. But, here's my reasoning at least.
The first idea I like for the simple reason that it can help give characters a boost in terms of prestige, and in turn the options of classes available to them. For example, unless the BT invasion transfers to EC in the near future, then unless something major happens in the inter-realm political situation, there's not going to be much fighting bar the odd monster or undead spawn. Yes, yes, I know people will say it's Battlemaster and you should do it through fighting, but it gives another avenue which as Tom has said, can also mean it is lost through fighting (by way of a takeover). Plus, if you've invested gold into your estate, and I'm assuming it would cost a lot to get up to the "vast" level, you're less likely to leave it. Of course, your Lord could be a complete ass and kick you, meaning that money is wasted. So it's a gamble - risk and reward.
As for the idea of buildings, I've always thought that my various estates would have a few places which lend themselves to what they've accomplished. For example - Ravier and Malos are military leaders, so having a training or sparring ground for them on their estates makes sense. Viktor is an aspiring Priest - maybe he would have a small shrine to his faith which confers some kind of small religious bonus to the region. Alaron of course is a Lord - so having things like a vault again would make sense in an IC context of sorts. Yes, it would mean that things are decentralised but that could add a sense of extra strategy to fighting. If you know that one of the nobles in a region was funding the war effort by sending messengers from his vault away from a city, would you walk by and ignore it, or would you stop this line of funding by sacking the region? That in fact, is another point - if you have these buildings on your estates, it could be made that sacking a region also has an effect on any estate with a vault, or something similar in the case of other buildings or add-ons.