No 13084, Wednesday 04 Nov 2020, Lightning

Solution to 8D has been deliberately left unsolved and is to be answered only by a non-regular / novice commenter, with proper annotation. Those who have answered earlier in the week, please give others a chance. 

ACROSS
1   Overpowering fear about backing medicine for widespread illness (8) PANDEMIC {PAN{MED<=}IC}
5   Mostly calm, appeal with resigned expression (2,2,2) SO BE IT {SOBEr}{IT}
9   Act grand right away in fear of violent battle (8) DOGFIGHT {DO}{G}{FrIGHT}
10 Seed that is on headgear (6) BEANIE {BEAN}{IE}

12 Flat helmet holding tool (5) LATHE [T]
13 Rebuilding pier included in dull plan (9) BLUEPRINT {BLU{PIER*}NT}
14 Everyone participating in fancy dance (6) BALLET {B{ALL}ET}
16 Wild flaw lessened in part (7) LAWLESS [T]
19 Argue about religious education cost (7) FREIGHT {F{RE}IGHT}
21 Trade lawyer disheartened (6) BARTER BARrisTER
23 Absolutely low on alcohol at gathering (9) LITERALLY {LITE}{RALLY}
25 Engineer carried by own lift (5) HEAVE {H{E}AVE}
26 Errors in printing? Tear a royal arrangement (6) ERRATA {TEAR+A+R}*
27 Contain settlement leaving out mantra (8) COMPRISE COMPRomISE
28 Strap human essentially in a state of shock (6) TRAUMA {sTRAp}{hUMAn}
29 Lawyer vacillating on treaty (8) ATTORNEY*

DOWN
1   Bat with power and stupefy (6) PADDLE {P}{ADDLE}
2   Mending one hospital garment is an ordeal (9) NIGHTMARE {1+H+GARMENT}*
3   Nearly leave the Parisian in forced absence (5) EXILE {EXIt}{LE}
4   Stay at home by practice (7) INHABIT {IN}{HABIT}
6   Subdue and cut short open authority (9) OVERPOWER {OVERt}{POWER}
7   Penny quit cutting corners in boredom (5) ENNUI {pENNy}{qUIt}
8   Formal study to consider institute at Southern Spain (8) T?E?T?S?  (Addendum - TREATISE {TREAT}{I}{S}{E} - See comments)
11 Almost stocked, saving energy with petrol, say (4) FUEL {FU{E}Ll}
15 It could provide power to record, and harm one tenor by mistake (9) LOGARITHM {LOG}{HARM+1+T}*
17 Joy over English victory, going to a high place (9) ELEVATION {EL{E}{V}ATION}
18 A fellow’s eloquent and wealthy (8) AFFLUENT {A}{F}{FLUENT}
20 Not commencing old story (4) TALE sTALE
21 Refuse to deal with opener in the past (7) BOYCOTT [DD]
22 Setter replaced a handy fix (6) REMEDY RE(-a+me)MEDY
24 Drink around river right by land (5) TERRA {TE{R}{R}A}
25 Animal held in prison pounced on leaders (5) HIPPO Acrostic

Refereence List

Appeal = IT, Grand = G, Right = R, Religious education = RE, Engineer = E, Royal = R, Mantra = OM, Power = P, Hospital = H, Institute = I, Southern = S, Spain = E, The in Parisian = LE,  Energy = E, Tenor = T, English = E, Victory = V, Fellow = F, River = R, Right = R