It is very likely Bitcoin will never have a "split.
Of course there is a "top". The big exchanges and brokers, along with the core developers.
Two years ago MtGox could have single-handedly switched the denomination and everyone would have updated their services. Right now there are more big players, but it would only take half a dozen influential people to make the switch happen in a week.
No it wouldn't. My client rejects blocks which don't match the correct Bitcoin protocol. There are 21M BTC, the current subsidy is 25 BTC and coins are divisible to eight places. So you are saying if you get all the major cexchanges, service providers, plus wallet developers for all the wallet to all simultaneously change you could force a change. Even that is incorrect but sorry to break it to you but all those people in that group don't agree on just about anything. Hell they often don't agree on very non-contraversial changes. However you believe by magic they will all agree simultaneously on probably the most controversial change to Bitcoin ever? Really?
When you have to get dozens of people from diverse views with diverse viewpoints and often conflicting agendas to reach a consensus and then gets hundreds of thousands of users to upgrade their software to make that change effective that is by definition "no top".
I thought everything at the protocol level was processed at the satoshi level (eg zero decimal places) although perhaps this varies between software...? I'm sure you are more of an expert here than me.
But anyway, all the "split" would do in this instance is to 'rename' "one bitcoin" to equal 100 satoshis instead of 100 million (or 100,000 instead of 100 million).
I have to admit, I am all for this. The decimal place is horrible for 99.999% of people; try asking anyone what even a simple sum such as 0.001 + 0.0001 is verbally and I guarantee that they will struggle (i have tried this on many [non-technical] friends myself). Also then ask them to convert this into their native currency by multiplying by, even a round number such as 500. Not happening... Multiply those numbers by a million and people have no problem though.
It is my opinion that mBTC will not solve this problem adequately either; merely further delaying the issue until a point in time when we have more (less-technical) users and more nodes which will have to consent.
My vote would strongly urge to adopt 1x10^-6 (current) BTC in some form, either uBTC (less desirable) or the "stock split" - changing BTC to equal 100 satoshis (most desirable).
I am sure if this was agreed to by a majority in the communtity that a date or block could be set as changeover day to attempt to minimise confusion, which would inevitably follow, breifly. Although with adopting 1x10^-6 BTC the difference should be so incredibly obvious (who will pay 1,000,000x too much for their service!?) that there should not be much issue...