Some of the answers are not straightforward and require some mental gymnastics.
1 is easy. He was a cleaner of locomotive engines, de-sooting and decoking the engine on its servicing schedule.
2 He didn't actually do 3 active and 3 in Reserve. He did 1 day active, which confirmed him as having signed up for military service, and was transferred to the Army Reserve on his 2nd day. This was procedural, and entirely normal for this class of enlistment. Some background.
The UK had a standing army of many different services garrisoning home and the far flung empire. The number of active serving men required in times of relative peace was very much less than what would be required in the event of a major war. The Army Reserve was the means of rapidly filling the extra requirement. While on Reserve a man was free from military law and have employment at home or abroad but was still liable to be recalled for army service on mobilization of the Army Reserve. On mobilization or recall to army service the man was immediately subject to military law. If he didn't respond to being recalled he would be classed as ABSENT WITHOUT LEAVE or even as a Deserter.
Henry's attestation papers have a special heading related only to enlistment in the POST OFFICE CORPS, and ROYAL ENGINEERS (Telegraph Reserve, Railway Reserve, and Submarine Mining Reserve). These men had a special arrangement with the military, active service 1 day then draw Army Reserve pay for 6 years, BUT BE AWARE THAT WE MIGHT CALL YOU OUT AT ANY TIME IN THAT 6 YEARS TO SERVE ANYWHERE IN THE WORLD (and if you are serving abroad when the 6 years is up, then we can keep you for another year.) Your man in July 1899 never dreamed that he would have to answer the call to arms so quickly.
3 is what it says
4 is what it says. Yes he came back to UK on 23 Aug 1901 and was retransferred back to the Reserve on 15 May 1902, being finally discharged 23 July 1905.
The majority of the Railway Reserve served in SA right through to 1902, thus earning a KSA to go with their QSA. That Henry returned to UK in 1901 suggests that he was unwell from disease.
It is a very complicated story but it is thoroughly explained in a book already mentioned "FROM CREWE TO THE CAPE". Well worth getting hold of to really get to grips with the Railway Volunteers Get the book, read it, inwardly digest, then come back with any questions arising.
Pete