Hi DownUnder,
1. I take it the details you quote re your ancestor at Princess Royal came from the Records at the Dean Heritage Museum wrt your earlier thread ?. If so then I would suggest they should be able to help clarify these details you mention.
This forum has had contributions over the last few years from people directly involved with mining so far more qualified than most to answer your question. However sadly I've not seen them on the forum for some time. One such was "Vurrister" as can be seen from this old thread. Hopefully if you email him direct via his username icon he'll be able to help you out - I do hope so, he kindly helped me offline back then.
http://www.forest-of-dean.net/index.php/forum

My guess is the numbers you quote refer to his precise place of work within the colliery. As you've read from this http://www.lightmoor.co.uk/forestcoal/CoalPrincessRoyal.html
Princess Royal was a huge colliery with "roads" extending many thousands of yards from the shafts out to the working coalfaces. I've seen the term "roadsman" used about mining during Victorian times,
eg from the glossary within this excellent mining history website http://www.scottishmining.co.uk/3.html , found as so often with mining queries from the superb http://www.cmhrc.co.uk/site/home/index.htm

"ROAD, A. passage in an underground working; a working place ; a way by which mineral is got out.
ROADHEAD, the end of a road at the working face.
ROADSMAN, or REDDSMAN, An underground official who attends to roads, laying rails, &c."
It's my guess the term "roadmaster" is a more modern/local variation of "roadsman", or perhaps he was the master in-charge of the various roadsmen at the colliery; each road or even part of one being under the local control of a roadsman responsible for it's daily operation & maintenance etc.
Hopefully someone with mining experience can help out.

2. Re West Dean, until very recently this was the official name for the local government area/parish which Bream is within. The Genuki site below helps show these various areas and how their boundaries and names changed with time. If you've used Ancestry you may have been confused as I was when that site, like others, sometimes confuses us with the "West Dean" area within Sussex.
http://www.ukbmd.org.uk/genuki/reg/districts/forest%20of%20dean.html
http://en.wikipedia.org/wiki/West_Dean,_Gloucestershire

Re your "poor" geography, dont forget this site carries some useful maps, this will help as a start, found under the "Forest of Dean" section of the newlook site.
http://www.forest-of-dean.net/index.php/resources/15-maps/98-forest-of-dean-west-circa-...

3. As MPG has suggested, the most likely "mill" in Lydney is the tinplate works (your references appear to be too recent to suggest a watermill, for example). It's hard to believe now but in past times even picturesque Wyeside Lydbrook was a hive of industrial activity, including a tinplate works among forges and pits, this was linked to the similar one at Lydney which had the advantage of being near the more navigable Severn.
http://en.wikipedia.org/wiki/Lydbrook#Industry
The plate was produced by repeatedly passing white hot metal between steel rollers to gradually reduce it's thickness to the required gauge, hence this part of the works would be called a mill as with a steel mill.
http://www.forest-of-dean.net/index.php/lydbrook
http://www.forest-of-dean.net/gallery/lydney/pages/page_22.html

This site describes the Lydney "millmen" at work
http://www.sungreen.co.uk/Lydney/Tinworks.htm
http://www.sungreen.co.uk/Lydney/TinplateWorkers.htm
I have an excellent little book which has similar graphic depictions of the hard working life at both the Lydbrook & Lydney tinplate Mills in their later years, see http://forum.forest-of-dean.net/index.php?id=38178

From the ever-reliable British History site
"By 1844 James was using Lower forge as a tinplating works. His family surrendered the lease in 1847 and a new one was granted to members of the Allaway family, ironmasters and tinplate manufacturers. In 1864 W. Allaway & Sons were producing c. 1,000 boxes of tinplate each week as well as some sheet iron, and they were employing c. 400 workers, most of them from Lydney parish where the works remained the principal source of employment until the mid 20th century. From 1876 the works were leased to Richard Thomas, who already occupied tinplate works at Lydbrook. In 1889, after the firm had spent £6,000 on improvements and new machinery at Lower forge, it took a new lease which empowered it to remove the machinery from the three upper sites and drain their ponds, and the upper sites were abandoned soon afterwards as manufacture was concentrated at Lower forge. Richard Thomas & Co., in which Richard was succeeded as managing director by his son Richard Beaumont Thomas in 1888, became one of the principal tinplate manufacturers in the country, acquiring other mills in South Wales. In 1941 the Lydney works closed but they reopened in 1946 under the name of Richard Thomas & Baldwin. With the nationalization of steel in 1951 the works became part of the Steel Co. of Wales and continued under that style until they closed in 1957."

From: 'Lydney', A History of the County of Gloucester: Volume 5: Bledisloe Hundred, St. Briavels Hundred, The Forest of Dean (1996), pp. 46-84.
URL: http://www.british-history.ac.uk/report.aspx?compid=23251