Do you still need a solution for this?
remove <?php $this->loadBlock('mainnav') ?> from default.php
add to header.php

<header id="t3-header" class="container t3-header">

	<div class="row">



		<!-- LOGO -->

		<div class="col-xs-12 <?php echo $logosize ?> logo">

			<div class="logo-<?php echo $logotype, ($logoimgsm ? ' logo-control' : '') ?>">

				<a href="/<?php echo JURI::base(true) ?>" title="<?php echo strip_tags($sitename) ?>">

					<?php if($logotype == 'image'): ?>

						<img class="logo-img" src="/<?php echo JURI::base(true) . '/' . $logoimage ?>" alt="<?php echo strip_tags($sitename) ?>" />

					<?php endif ?>

					<?php if($logoimgsm) : ?>

						<img class="logo-img-sm" src="/<?php echo JURI::base(true) . '/' . $logoimgsm ?>" alt="<?php echo strip_tags($sitename) ?>" />

					<?php endif ?>

					<span><?php echo $sitename ?></span>

				</a>

				<small class="site-slogan"><?php echo $slogan ?></small>

			</div>

		</div>

		<!-- //LOGO -->

	<div class="col-xs-12 col-sm-8">

    <?php $this->loadBlock('mainnav') ?>

    </div>



	</div>

</header>

modify mainnav.php
change <div class="container"> to <div class="row2">