There are some things to bear in mind...
If your end of employment is within 15 weeks of your EDD, you are entitled to your SMP if your employment terminates. They are not doing any favours by giving you this, they have to!(see here.
SMP cannot be paid out tax free unfortunately, no matter when or how it is paid. This is because it is remuneration, so must go through payroll and be taxed. Only compensation (i.e. things that they do not have to pay you but choose to) and damages for breach of contract can use the £30k exemption.
In terms of payment dates, yes, I would get them to ring the Revenue and confirm that a lump sum is ok. Then you should be able to terminate your employment, have your SMP through payroll(taxed) and your redundancy (statutory redundancy, any enhanced) tax free. The treatment of your notice pay depends whether you work your notice period and also the wording of your contract (sometimes it can be tax free, sometimes not).
You are also right about the issue of getting another job and SMP. However, bear in mind that that technically applies however it is paid out. If you start a new job, you are no longer on maternity leave, so are not entitled to maternity pay from that point onwards. In theory, they could claim it back from you even if they go for a lump sum. I would make sure that they agree that the lump sum SMP is yours whatever happens (i.e. if you get another job during the nine months, you will tell them so that they won't unlawfully claim it back from HM Revenue, but they won't reclaim the money from you).
In terms of being pregnant and not on maternity leave, you are right that the special rights for protection against dismissal do not apply (those on maternity leave get first dibs on remaining vacancies). However, if you think you have been selected because of your pregnancy (or indeed your part time status), then that is a different matter. Come back to us if you're worried about that.