No, as evidenced by the several exchanges that have already done it (including a few more that aren't around any more).
Walletnotify/blocknotify are are APIs from bitcoin (and bitcoin forks) and when we support something closer to those (it likely won't ever be exact because of the inherent differences in the technology), it will become easier to integrate with systems already supporting those other coins. But without them it can obviously still be done, just with a bit more work.
Ok well then in terms of question Number 2. At $0.70-$0.75 isn't Monero under priced if it were to ever be used in lets say an underground market for example? And what other barriers would there be to implementation of Monero in such a marketplace if any?
The only right way to think about if something will go up or down is to think it from the perspective of supply/demand intermediation.
Supply we know from the parameters. Then you ask yourself a question will there be more propensity to hold coins than today. Note, if there are 2 guys holding each coin for 1 week and one coin per person, it means one coin is held 2 weeks (for instance, if I am paying my rent with 1 xmr after holding it for one week and the landlord of my appartment is holding the coin for 1 more week before cashing it out, the coin is held 2 weeks, but in the hands of different persons).
Therefore, creating chains of holders is important for developing the Monero economy after we have some kind of merchant acceptance.
Now we still are in the age of speculation so we need to think if the willingness to hold a coin will decrease or increase from the current level. Personally I think the price of the coin doesn't matter wether it is under/overpriced, all that matters is, will there be more held coins vs. emitted coins in the future than there is now.
I am optimistic in this and therefore I am holding. I am thinking there is relatively high chance of higher amounts of coins held than there is now. If I didn't think in this way I would have dumped (all) my coins long time ago.
Even if Monero fails in getting merchants, it doesn't mean it wouldn't succeed better than bitcoin. Monero is kind of better coin for storing wealth, in this it is a little bit like UNO but with bigger community.
The network-effect and adoption will happen by raising the price little by little all the time and making sure, the long term holders will not be at loss. That will create satisfied salesmen for the coin and the price can rise exponentially.