No.12212, Thursday 11 Jan 2018, Scintillator

Typically good and challenging puzzle from Scintillator. Especially liked IN SPADES, SNOOZE, COLD CALL, LOCUSTED, PULL-TAB, ENGAGED, EXCITON.

I have a feeling some parts of the clues for 20A and 23A are mixed up (..is reflected... from 20A for example explains part of the wordplay for 23A).  I stand corrected. See comments below

But continuing the trend started by Colonel, I'm leaving those two answers open to everyone for discussion.

Newish commenters can have a go at identifying the theme and the associated words. I'll amend the blog later in the day with the updates.

ACROSS
1 Dispersive elements in good message blocked by religious head (6) PRISMS [PI(=good) + SMS(=message) outside Religious]
4 Willing to cut breaks before end of period (8) PREPARED [PARE(=cut) inside PRE(=before) + perioD]
9 Leave undisturbed and be late to school (3-1-2) LET-A-BE [BE LATE]* LET BE is more familiar, but Chambers lists LET-A-BE as a dialect
10 Extremely pained sympathisers, extremely agitated (2,6) IN SPADES [PAINED SympathiserS]*
12 Somersault-doing model's lungi dances about right (8) TURTLING [T(=model) + LUNGI* outside RighT]
13 Drop off tin with leak (6) SNOOZE [SN(=tin) + OOZE(=leak)]
15 Blood pressure expands partially — "That's a bad word!" (4) BEEP [BEE P(=bp partially expanded)]
16 On return, first man needs oxygen cylinder, possibly an African (10) MADAGASCAN [ADAM(=first man)<- + GAS CAN(=oxygen cylinder)]
19 He must be worried about healthy old man (10) METHUSELAH [HE MUST* + HALE(=healthy) <-]

20 Mood is reflected being out every so often (4) ?O?E TONE  {bEiNg+OuT}<= See comments below
23 Mum, the empty tummy remains empty during this time (6) S?L?N?  SILENT {SI}{LENT} See comments below
25 Japan and Russia at war about 150 million years back (8) JURASSIC [Japan + RUSSIA* + C(=about)]
27 Reserved name as a sales tactic (4,4) COLD CALL [COLD(=reserved) + CALL(=name)]
28 Some gasbag has terrorists stunned (6) AGHAST [gasbAG HAS Terrorist]
29 Hundred in place of one — solitude badly destroyed (8) LOCUSTED [SOLITUDE with C for 1]*
30 Verbal from nose-poking players (6) SPOKEN [POKE(=nose) inside SN(=players)]

DOWN
1 Cricketing shot key for opener (4-3) PULL-TAB [PULL(=cricketing shot) + TAB(=key)]
2 One percent time spent to snoop on others (9) INTERCEPT [1 PERCENT Time]*
3 Fluid (doctor's fluid) (6) MOBILE [MO(=doctor) + BILE(=fluid)]
5 Get to unwrap box here? (4) RING [bRING(=get)]
6 Dandy having soda at home with Juliet (8) POPINJAY [POP(=soda) + IN(=at home) + JAY(=Juliet)]
7 Wireless port has a drive at one end plugged in (5) RADIO [RIO(=port) outside A + Drive]
8 Make out research institute is close to hell (7) DISCERN [DIS(=hell) + CERN(=research institute)]
11 Busy, // having committed to wedding (7) ENGAGED [DD]
14 Bachelor's tricked, your Honour! (7) BAHADUR [BA(=bachelor) + HAD(=tricked) + UR(=your)]
17 Angry gossip or witty conversation? (5-4) CROSS-TALK [DD]
18 Hot objects with sides featuring a vase (8) FURNACES [FACES(=sides) outside URN(=a vase)]
19 Turnover of hundred thousand is er pleasing to the ear (7) MUSICAL [LAC(=100 000) + IS + UM(=er)] <-
21 Old rum tonic is a source of energy (7) EXCITON [EX(=old) + TONIC*]
22 Gun shot during accident causes difficulty (4-2) HANG-UP [GUN* inside HAP(=accident)]
24 Tree mounted in botanical illustration (5) LILAC [botaniCAL ILlustration] <-
26 Changing hands to unrestrain bolt (4) FLEE [FREE(=unrestrain) with L for R]

GRID