Solution to 28A 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 Mushroom backed by Gussalufz — cracking recreational material for smoking device! (8) PIPECLAY {CEP<=}{I} in {PLAY}
9 By going outside and enduring... (7) ETERNAL ExTERNAL
10 ...wind, Bertie had clipped laziness (7) INERTIA {wINd}{bERTIe}{hAd}
11 Grab energy in bulk (5) SEIZE {S{E}IZE}
12 What's that old rotten scum hiding? European ecstasy? (6,2) EXCUSE ME {EX}{CUS{E}M}{E}
13 Statement of customs dues (6) RIGHTS (Correction (~rites) - See comments)
18 This process takes three to nine seconds, as reactivity is not great initially (8) SQUARING {S}{QUA}{Re...y}{Is}{Not}{Gr..t}
20 Adventurous, free-thinker alien going for love (6) HEROIC HER(-et+0)OIC
23 My companions and I intend to drink primarily, parting very loaded (4-2-2) WELL-TO-DO {WE'LL}{TO{Dr..k}O}
27 Drunkard returns drinks (they are unpredictable) (4-3) TOSS-UPS {SOT<=}{SUPS}
28 Non-stop liquor leads to trouble? Exactly, that's related (7) C?G?A?E (Addendum - COGNATE {COGNAc}{Tr...e}{Ex...y} - See comments)
29 Deeply, loudly rejected dropping regulars and calmed down (6) LULLED {DeEpLy+LoUdLy}<=
30 Chant quietly treasured by retired antique person like me (8) COMPILER {OM}{P} in {CILER<=}
DOWN
1 Priest comforts losing American forces (9) PRESSURES (Correction {P}{REaSSURES} - See comments)
2 Upset Nepali girl making an appeal (7) PLEAING {NEPALI*}{G}
3 Transformative device essentially planned in secret room with critical care (9) CONVERTER {CO{plaNned}VERT}{ER}
4 A student, idle, rolled joint (6) ALLIED {A}{L}{IDLE*}
6 They shoot infiltrating forces, resulting in retaliation (5) USERS [T<=]
7 Perfect parent supporting pot freely? Yours truly! (7) OPTIMUM {MUM}<=>{POT*}{I}
15 Busy penguin's put in work for accelerating (7,2) OPENING UP {PENGUIN}* in {OP}
16 Klepto periodically involved in dodgy career as conman (9) RACKETEER {RAC{KlEpTo}EER*}
17 Ambassador's very put out by Judge's fancies (6) ENJOYS EN(-v+j)JOYS
19 Device blocks signal — is net usage up? (7) UTENSIL [T<=]
21 Weird, barmy old lad embodying Britain (7) ODDBALL {OLD+LAD}* over {B}
22 Secretly chomp on chorizo wrap (6) PONCHO [T]
24 Short tough drill (a watchmaker's tool) (5) LOUPE {LOUt}{PE}
Reference List
By = X(Into), Energy = E, European = E, Ecstasy = E, Seconds = S, As = QUA, Alien = ET, Quietly = P, Priest = P, American = A, Girl = G, Student = L, Rook = R, Bishop = B, Very = V, Judge = J, Britain = B, Drill = PE