You can try this way if it works for you. Its just a fixed.
1) Override module-rma/view/frontend/templates/form/renderer/select.phtml in your theme or module. add data-validate="{required:true}" for required attributes.
select.phtml.
<?php
/**
* Copyright © Magento, Inc. All rights reserved.
* See COPYING.txt for license details.
*/
// @codingStandardsIgnoreFile
?>
<?php
/**
* Create account form template
/* @var $block \Magento\Rma\Block\Form\Renderer\Select */
$validate = ($block->isRequired()) ? 'data-validate="{required:true}"' : '';
?>
<div class="field <?= $block->getHtmlId() ?><%- data._index_ %><?php if ($block->isRequired()) {
echo ' required' ?>">
<label for="items:<?= $block->getHtmlId() ?><%- data._index_ %>" class="label"><span><?= /* @escapeNotVerified */ $block->getLabel() ?></span></label>
<div class="control">
<select id="items:<?= $block->getHtmlId() ?><%- data._index_ %>" name="items[<%- data._index_ %>][<?= /* @escapeNotVerified */ $block->getFieldName() ?>]"<?php if ($block->getFieldName() == 'reason'): ?> data-args='{"index":<%- data._index_ %>}' <?php endif;
} ?> <?=$validate?> >
<?php foreach ($block->getOptions() as $option):?>
<option value="<?= /* @escapeNotVerified */ $option['value'] ?>"<?php if ($option['value'] == $block->getValue()):?> selected="selected"<?php endif;?>><?= $block->escapeHtml($option['label']) ?></option>
<?php endforeach;?>
<?php if ($block->getFieldName() == 'reason'): ?>
<?php $attribute = $block->getAttribute('reason_other'); ?>
<?php if ($attribute): ?>
<option value="other"<?php if ($option['value'] == 'other'):?> selected="selected"<?php endif;?>><?= $block->escapeHtml($attribute->getStoreLabel()) ?></option>
<?php endif; ?>
<?php endif; ?>
</select>
</div>
</div>
2) For second issue please add below html before fieldset ends in create.phtml.
<div class="actions-toolbar text-right">
<div class="secondary" id="delete<%- data._index_ %>">
<button type="button" class="action remove delete"><span><?= /* @escapeNotVerified */ __('Remove') ?></span></button>
</div>
</div>
3) For third issue I think if point 1 resolved from this fix then you won't face this issue anymore.
I hope it will work for you, Let me know if you face any further issue from these fixes.