How could an LN node acquire inbound liquidity? Is there any systemic risk? I once heard that LN could support dual-funded channels. It doesn't seem so complicated, but currently, I don't know any LN wallet which supports creating dual-funded channels.

By the way, I'm also concerned about security: will dual-funded channels introduce systemic risks/attack vectors? For example, if an attacker is already able to do a 51% double-spending attack, could he leverage the inbound liquidity to multiply his profit?
Dual funded channels are theoretically easy but there is a little bit of implementation overhead. Also they are not fully spect out yet in the BOLT standard which is the reason why they are not implemented yet. They will be part of the next update to BOLT 1.1 and people are working on the specification.

While studying the Lightning Network white paper last year I discovered a potential 51% attack vector which I described on the Mailinglist. The attack can steel all the capacity that is in all your channels. With dual funded channels you could in fact increase the capacity that is attached to you. However the described 51% attack is rather unlikely to happen and in particular would yield many other problems.