Fifteensquared

This one of those puzzles that appeared quite daunting at first pass, but ended up being  plainer sailing than initially feared.  Mr/Mrs/Ms Phssthpok can usually be relied upon to inject more than their fair share of clue-writing originality into proceedings, as is the case today.  However, I did feel that several of the clues were a little less polished than is usually the case.

Across
1 SOCIAL NETWORKS - work (=’have guts’, in the mechanical sense) in coastline*.
10 ONION – on + I + on.  Purist couldd justifiably query the appearance of ‘under’ in an across clue.
11 GOALMOUTH - go + {F}almouth.  A clue of Kinnockesque verbosity, where both the surface and the definitional sub-component still seem a little unsure of themselves.
12 TWITTER – w[ife] in titter.
13 SANCTUM - (man’s cut)*.
14 SCALE – double def’n.  The ‘need’ is purely for surface padding.
16 THRESHOLD – thresh + old.
19 PREFORMED - performed, with the second and third letters swapped (i.e. a ‘single exchange’).
20 RABID - b[lack] in raid.  Anyone else remember the character “Johnny”, from The Fast Show, who would always end up painting over his artistic works-in-progress with black?  I hadn’t for years, but the ‘Crazy black…’ intro to this clue, somehow sprang him unexpectedly from the junk-pile recess of my brain.
22 GUFFAWS – guff + w[icket] in as.
25 ARRAIGN – a + homophone of “reign”.
27 SUMMING UP - double def’n.
28 LEAST – homophone of “leased”.
29 SURGE PROTECTOR - double def’n; each related to a different kind of current.

Down
2 ORIGINATE - (I + gin) in orate.  A tricky looking clue, that turned out to be a fairly oft-deployed wordplay combo.
3 INNIT – inn + it.  One of those marmite (“love it or hate it”) clues, I suspect.  Personally, I give it the thumbs up.
4 LOGARITHM - log + a + homophone of “rhythm”.  The definition is admirably innovative, as we rarely see mathematical answers clued as anything less banal than ‘function’ or ‘constant’.
5 EXAMS – axe< + MS (‘manuscript’).
6 WOMANISER - (0 + man) in wiser.  Not a meaning of the word ‘wolf’ that I’d encountered before, but it makes perfect sense, given that such individuals are often referred to as being “on the prowl”.
7 ROUST – US in rot.
8 SCHEMED - s[uccess] + chem{ist} + e{scape-po}d.  Stylistically, this one fell into the same camp as 11A for me.
9 MONTHS – N in moths.
15 EVOCATIVE - (Evita + cove)<.  Neat and tidy, tidy and neat.
17 RED CARPET - D[iamond] in re-carpet.  I think… although that interpretation is dependent on ‘with’ being an insertion indicator.
18 OBBLIGATO -(bal{l} + too big)*.
21 DONATE - don + a + te.
23 FEMUR - em in fur.
24 SUGAR - initial letters of ‘solution unblocks gastric and renal’.
26 RELIC - Eli in RC (Roman Catholic).