Problems with Video Embeds

Hi Folks,
I have just updated this week from Weaverii to a Xtreme 4.2.6 child.
Generally, all is working okay, but I have found a few issues regarding videos which I urgently need to fix and ask for advice and help.

1) When I insert a YouTube video into a post, I use the default settings and then center it. e.g.:

‘<p style=”text-align: center;”><iframe src=”//www.youtube.com/embed/Updkb3Bfkrw” width=”446″ height=”250″ allowfullscreen=”allowfullscreen”></iframe></p>’

However, though it shows correctly centered and 446×250 on the edit page, when viewed on the page, the video is full width.
This affects all old videos with this form of embedding and messes up a whole lot of pages.
How can I get back the old page view of a centered 446×250 video?

2) Some of the older (2010+) videos used a different embed procedure:
‘<div style=”float: left; margin-right: 14px;”><iframe src=”//www.youtube.com/embed/wivgDck1l88″ width=”243″ height=”200″ frameborder=”0″></iframe></div>’
All such embeds no longer work. They show no videos at all – or spaces for them – on the post, only the page text.

However, if I delete the ‘<div>’ positioning, and use only ‘<iframe src=”//www.youtube.com/embed/wivgDck1l88″ width=”243″ height=”200″ frameborder=”0″></iframe>’
Then the videos shows on the page – but again, it is full width and so destroys the page layout and makes pages unreadable.

Can you please advise a solution, or where to look for one, so I can have predetermined sized videos placed where I need them, in and amongst text?

Regards and thanks,
Angus