I realise your post was directed at Marc 1 but I decided to give my thoughts on the subject.
If a second country seriously wanted to attack singapore:
For a start, if neighbouring countries wanted to do as you indicated, they would have to significantly increase their defence budget, which would not be possible in the short term as it would be damaging to the national economy as a whole. After that, they would need time, and lots of it, to train and develop the skills and support infrastructure needed to operate beyond their borders, in high tempo, extended operations. And they would be operating against the Singapore Armed Forces [SAF], which unlike the Malaysian Armed Forces [MAF] and theTentera Nasional Indonesia [TNI] has been able for several decades to focus on external security, is much more networked, has numerical superiority in many key areas and unlike the MAF and the TNI, has much smaller operational peacetime responsibilities, due to Singapore being a much smaller country.
To date, neither Malaysia or Indonesia has taken any steps to ensure that it has the capability to threaten any other country in ASEAN - for the reason that they do not have to - and this is unlikely to change anytime soon. And as I mentioned before, the MAF for the past few decades was focused on counter insurgency, not in external security - the same goes for most ASEAN countries - and this is still reflected in the way their armies are organised and equipped.
SRBMs and MLRS systems are readily available and are tried and tested technology - thus they are effective. These systems can be bought easily and quickly - sometimes off shelf or even from seller countries' reserve stock.
Yes they are readily available but haven't been taken up by any ASEAN countries due to political reasons and because they might not be very useful things to have, given the threat perceptions of various ASEAN countries. This is not the Middle East and things haven't reached a point where ASEAN countries would want to acquire ballistic missiles.
The very fact that Singapore has sought ABM defences indicates that they themselves have considered this threat as credible.
We can speculate as to the reasons why Singapore got the Iron Dome. Many on various forums and blogs are of the opinion that it was to counter Malaysia's ASTROS. My personal opnion is that it was to cater to the possibility that certain countries in the near future might acquire cruise missiles such as Brahmos. Anyhow, the whole of SEA is well within range of Chinese ballistic missiles and has been for some time now.
would easily notice a build up of missile and artillery forces - questions is what would they do about it? Initiate pre-emptive strikes against dispersed targets?
If things had reached a point where diplomacy had failed and ASEAN, the UN or the U.S. had been unable to defuse tensions, the leadership of Singapore - if convinced that hostilities were imminent and that there was a clear threat to Singapore - would have no choice but to launch a pre-emptive strike.
The five power agreement is an unknown quantity - and one can only speculate on how that would play out once hostilities ensued.
It is not an unknown quantity. It role is clearly defined and it provides a platform where discussions can be undertaken as to how to proceed in event of an external threat on Malaysia and Singapore. It replaced the Anglo Malayan Defence agreement [AMDA] and does not compel the U.K, Australia or New Zealand to come to the aid of Malaysia and Singapore, as it is not a binding alliance like NATO.
In event of a full scale conflict involving Malaysia and Singapore, the FPDA might be able to play as role as a mediator, alongside ASEAN, apart from that there is not much it can do as it was not intended to cater for such an eventuality. As part of the FPDA, there are staff from all 5 FPDA members [including several Republic of Singapore Air Force officers] attached to the IADS HQ [commanded by a Australian officer] in Butterworth, a liasion officer from the Royal Malaysian Air Force at Changi, a Royal Australian Air Force presence at Butterworth and a Rifle Company from the Australian army permanently deployed at Butterworth, so achieving strategic surprise will be a bit hard. And as part of the FPDA, members also share intel under the ANZMIS arrangement.