Vielen Dank, ich konnte es mit einer Theme-Komponente fast zum Laufen bringen:
{{~raw-plugin-outlet name="topic-list-header-before"~}}
{{#if bulkSelectEnabled}}
{{#if canBulkSelect}}
{{raw "flat-button" class="bulk-select" icon="list" title="topics.bulk.toggle"}}
{{/if}}
{{/if}}
{{raw "topic-list-header-column" order='default' name='topic.title' bulkSelectEnabled=bulkSelectEnabled showBulkToggle=toggleInTitle canBulkSelect=canBulkSelect}}
{{#if showLikes}}
{{raw "topic-list-header-column" sortable='true' order='likes' number='true' forceName=(theme-i18n 'likes')}}
{{/if}}
{{#if showOpLikes}}
{{raw "topic-list-header-column" sortable='true' order='op_likes' number='true' forceName=(theme-i18n 'likes')}}
{{/if}}
{{#if topic.isPrivateMessage}}
{{raw "topic-list-header-column" order='posters' forceName='Participants'}}
{{/if}}
{{raw "topic-list-header-column" sortable='true' number='true' order='posts' forceName='Replies'}}
{{raw "topic-list-header-column" sortable='true' order='activity' forceName='Last Post'}}
{{#if bulkSelectEnabled}}
{{/if}}
{{~raw-plugin-outlet name="topic-list-before-status"}}
{{raw "topic-status" topic=topic}}
{{~topic-link topic class="raw-link raw-topic-link"}}
{{~#if showTopicPostBadges}}
{{~raw "topic-post-badges" unreadPosts=topic.unread_posts unseen=topic.unseen url=topic.lastUnreadUrl newDotText=newDotText}}
{{~/if}}
{{discourse-tags topic mode="list" tagsForUser=tagsForUser}}
{{#if expandPinned}}
{{raw "list/topic-excerpt" topic=topic}}
{{/if}}
{{#unless hideCategory}}
{{#unless topic.isPinnedUncategorized}}
{{category-link topic.category}}
{{/unless}}
{{/unless}}
{{~#if topic.creator ~}}
{{topic.creator.username}} {{format-date topic.createdAt format="tiny"}}
{{~/if ~}}
{{raw "list/action-list" topic=topic postNumbers=topic.liked_post_numbers className="likes" icon="heart"}}
{{#if showLikes}}
{{#if hasLikes}}
{{number topic.like_count}} {{d-icon "heart"}}
{{/if}}
{{/if}}
{{#if showOpLikes}}
{{#if hasOpLikes}}
{{number topic.op_like_count}} {{d-icon "heart"}}
{{/if}}
{{/if}}
{{#if topic.isPrivateMessage}}
{{raw "list/posters-column" posters=topic.featuredUsers}}
{{/if}}
{{raw "list/posts-count-column" topic=topic}}
{{avatar topic.lastPosterUser imageSize="medium"}}
{{format-date topic.bumpedAt format="tiny"}}
{{topic.last_poster_username}}
Es scheint, dass die Anweisung #if topic.isPrivateMessage für die topic-list-header nicht funktioniert, daher werden die PM-Teilnehmer-Avatare wie erwartet angezeigt, aber die Header fehlen ein <td>, sodass alles verschoben ist.