Is it really the case that there was a port of Microsoft Word and Excel that was one of the first really successful software to feature on a Apple device? Did Microsoft software actually have a surprisingly positive effect on early Apple products or was it's influence relatively minor?
Microsoft Excel was originally written for the Mac, and released in September 1985 (see Wikipedia article here). It was released for Windows 2 in 1987 (and was in fact bundled with a version of Windows).
Also, according to Wikipedia, Microsoft Word was released for the Mac "in 1985". It had previously existed as a DOS program, being released for the first time on Windows in 1989 (also on Windows 2).
I remember that Microsoft Word and Excel were really very good on the Mac in their early versions, but I don't think they had much of an impact on sales of the Mac platform overall. The initial "killer app" for the Mac platform was "desktop publishing" software. Aldus PageMaker was released on the Mac in 1985 (and on Windows in 1987), and QuarkXPress was released on the Mac in 1987 (and on Windows 3.1 in 1992).
My own recollection of the period was that the Mac was seen as essential in the 1980s if you needed to do desktop publishing - and that Windows didn't stand a chance at competing until v3.1 was released in 1991, because that version was the first that supported scalable TrueType fonts. This is because prior to that, Windows relied on raster-based fonts, that didn't scale - meaning that unless you had a copy of the typeface you wanted to use at the exact size you wanted to use it at, everything looked awful. The Mac had always been able to support the printing of PostScript fonts, and so had a head-start with making text look pretty when printed.
It seams as if the Question implies that there was some kind of fiendship between Microsoft and Apple. Such never existed. Microsoft was a major supplier of applications for all ranges of Apple computers and during all time.
Is it really the case that there was a port of Microsoft Word and Excel that was one of the first really successful software to feature on a Apple device?
No. At least not for Apple devices in general. Then again, for the Macintosh, Microsoft was a major supplier. In fact, Excel was developed for the Mac, only to appeared for the PC later.
Did Microsoft software actually have a surprisingly positive effect on early Apple products or was it's influence relatively minor?
Well, it started already early on. The Apple II's floating point BASIC called Applesoft, was a Microsoft Basic 2.0. So it came with each and every Apple starting with the II+.
Similar did Microsoft sell the Softcard with CP/M as base for their Z80 products. This shows quite well that there was no imagined shunning of each other, but cooperation. After all, CP/M wasn't a Microsoft product, but made by Digital Research. So Microsoft payed them royalties (as well as building hardware) to sell their products to Apple customers.
For the later Apple Microsoft was a launching partner, with Multiplan announced by Apple and Microsoft BASIC as some of the first products, already before the release of the Mac. Except, MS used the chance to do a complete redesign of Multiplan, as the code base was just out of hand (more than 100 parallel versions at the time), and replaced Multiplan soon. The new product was called Excel and targeted to beat Lotus 1-2-3.
In fact, Excel as well as Word development was for long streaks driven by the Mac version, not Windows.
It is known that Bill Gates was a great fan of the Lisa as well as the Mac. In fact, his original strategy was to get Apple to release a PC version of their OS, or at least licence it to Microsoft. Quite understandable, as having the same OS running cross platform (*1) simplifies application (and language) development a lot. Porting would be a matter of recompilation. MS was an Application and Language company, not an OS house.
MS had always a great interest in keeping Apple alive as Software for the Mac was a major source of income for MS. At some point they even owned Apple stock as part of a settlement - a cash injection crucial to Apples survival during the late 1990s.
Today Microsoft offers all their major products, from Office all the way to Visual studio - even the edge browser - in Mac versions. Most of them not just simple ports, but quite well integrated with the Mac UI language.
Wiki offers an overview of Macintosh software published by Microsoft.
*1 - Also a reason why MS tried to push Xenix as hard at the time.
I'm not sure you can get much earlier than Microsoft supplying Applesoft BASIC for the Apple II series in 1977.
It wasn't a port - the first versions of Excel came out on Mac, it didn't appear on Windows until version 2. The first version of Word came out on Xenix but it did appear on Mac very early on. I think it was more important in establishing Microsoft than selling Macs, as Lotus 123 and Mutliplan were the dominant spreadsheets back then.